If you do need them, any tire shop should be able to get them--maybe even Checker/Autozone type shops, too.
I've used 45 degree and 90 degree stems before, other rims don't require anything special. Depends on the location of the valve stem hole in relation to the inner ring.
This is what I did with mine but I got new short metal valve stems. Its a little tight on the front axle because of the hub but it works great. I just welded the stock holes up.