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!