for the orion quick set lathe you can get a diamond bit from mcmaster.com for like $64. p/n is 3316A42 if my recollection is correct and it uses an AL4 bit. i think the carbide that came with it should have the size stamped on it.
i've been using a mcmaster diamond bit for about 5 years now with 0 problems. they come in a bit long but can be shortened with a dremel and a cutoff wheel.