The 44T is just under 2.75" in diameter. I would buy the 3" or bigger just to be safe. 3" diameter is $25 per foot. I used a 1.3" (33mm) thick slice to make each pulley and retainer. You can check the sheet price if that works better on his CNC.
You will have to machine the pulley model form both sides, so see if he has a rotary table, or setup a jig for flipping the parts manually.
For the CNC I used, I was stuck using the 4" diameter which is $48 per foot. So twice as expensive, but easy to for me to hold in the machine. I can also get 2 retainer rings out of one slice, so not all bad. The way I bought it, I think I’ll get 5 pulleys and retainers for about $85. (I wanted to make 4, but bought stock for one extra for when I mess up the programming or milling)