WHere can I buy a 20" uni wheel?

Not a torker one from unicycle.com those probably won’t be strong enough… also would it be okay to put a 19" wheel on a 20" frame?
just broke mine today

What did you break? If it was the rim I would bring the hub to a bike shop, find a BMX rim, and ask them to build your hub into their rim. You might need to lend them your frame for the wheelbuild as unicycle wheels don’t fit in a standard wheelbuilding stand without adaptors.

Depending on how much clearance you have you might be able to fit a trials wheel in a 20" frame. Trials wheels were originally designed to fit in 20" BMX frames.

Most of the 20" unis use 17x40mm bearings. The splined trials hubs are 20x 42 for older stuff, and 22x 42 for newer ISIS hubs. So the trials bearing is 2 mm to big. Might bung in though, I don’t know.

Another trouble is the distance between the bearings. If that is ok, then will a 2.5 " tire fit between your 20" frame’s forks?

Because you can buy a Nimbus trials frame for 40 ish $, and it will be a lot stronger than your old 20 frame, you might want to look into that.