Can I build a wheel with two different spoke types?

In Roger Musson's book Professional Guide to Wheel Building 7th Edition, https://www.wheelpro.co.uk/wheelbuilding/book.php this is covered in brief (for a dished rear wheel):

The wheel can support more load before the left spokes go slack by making the right side spokes stiffer than the left and this is accomplished by using thicker (stiffer) spokes on the right and thinner (less stiff) spokes on the left.

But he also says any benefit is difficult to quantify and it's unproven if it makes a better wheel, before concluding:

I always use the same gauge spokes either side on dished rear wheels for the simple reason that the wheels work and I have no reason to look for a solution to a problem that does not exist.

It's a tiny section of a very thorough book, so not something to do routinely, but if you are looking to do this purely for spoke availability then yes you can if you were using the thicker spokes on the side with more tension.

In any other year I'd say don't bother, just persevere and find matching spokes from whichever seller (lots of online sellers can cut spokes down to length for you if you contact them) but 2020 is no ordinary year!

I have done it on purpose by using thinner spokes on the non-drive-side (NDS) of dished wheels. For example, I have a tandem with 2mm straight spokes on the drive side and butted 2mm/1.8mm spokes on the other side. The NDS naturally has lower tension, often about half the tension of the drive side. So if you can put the thinner spokes on the non drive side, it would be best. Some road wheels even have fewer spokes on the NDS than on the drive side. But even for a symmetrical wheel, though, mixing the spokes shouldn't be a major problem if the wheel is otherwise built correctly.

If there is a major difference, like 2.5mm spokes on one side and 1.8mm spokes on the other, you might have problems with the thicker being under-tensioned and coming loose. For my experience though, the difference between 2mm and 1.8mm is not enough to cause a problem; the characteristics overlap a lot.

Anecdotally I knew a guy who built his wheels with titanium spokes on one side and steel spokes on the other. I think this is asking for trouble because titanium has a much different spring constant that SS. But it worked at least for him.

Most of the wheels I've built have been mutts with random assortments of spoke types and not a one has failed, much less failed to come together. Have I built a wheel with a mix of bladed and round? Selected number of crosses to fit the length I have? Used 5 different spoke models? Yes. Often. Probably. So yeah it's definitely 100% possible. As for if it's a good idea, that depends on if you're building an ol' faithful 32 3x or trying something less resilient. In the latter case there's a lot to consider--relevant facts would be front or rear, wheel size, spoke count+lacing pattern, amount of center to flange asymmetry, rim strength, intended use, brake type, and your appetite for danger. But with 32 spokes and a near tangent line to the hub (typical of 3x on 26"+), all that really matters wrt strength and durability (provided you're not really pushing limits a la DH/BMX/trials) is that the spokes are tensioned high enough, and manually stress relieved at the end (else spokes may start breaking in a hundred or so miles). Practically speaking the more likely problems you're going to run into are:

1. breaking the Revolutions while trying to tension them because the ultra thin mid can't withstand a lot of windup, which is usually casually ignored with thicker round spokes (it's and obvious issue with blades, but lots of windup is never good--1.8/15ga+ spokes are just thick enough to resist it to an acceptable degree most of the time); ideally you'd clamp the spokes and use a tape flag or mark so you can visually confirm they're not twisting much while you tighten the nipples. To clamp I'd suggest making some soft jaws by filing small v grooves into some aluminum/brass/copper plates, and epoxying them to some good vice grips--definitely not needle nose, it requires a ton of force to stop a tiny ~polished rod from slipping, and needle nose have too much flex.
2. potentially running out of thread on the Revolutions before you reach the ideal tension. Reduction in diameter means an exponential reduction in area, and another exponent in volume; the revolutions are going to have a lot more give and thus require more thread to equal the force of the other side. In that situation you may be able to loosen the other side; you're sacrificing ultimate strength for a centered wheel, but tbh I've seen people riding wheels where the spokes were very nearly slack, as in some nipples could be turned by hand, so ultimate strength and the amount you need are different things. Did you see the meme video of the cop in the skatepark bowl where his front wheel collapsed? Those spokes were way too loose, probably always were, but under his normal use with straight down load it was "fine". As soon as a little side load got thrown into the mix: faceplant taco.
3. self loosening nipples? Unlikely if you can get acceptable tension; really if under load the tension drops enough to allow the nipple to budge, you're probably closer to system failure (taco status) than you want to be. You could use that as an indicator, or use blue threadlocker or spokeprep if you don't want to know.