honestly, if you want to get it done you have to design around what you can get. Sometimes they sell different belts in your country for whatever reason. The only thing that you need to keep in consideration is the 5mm shaft for the motor. Everything else can be adapted. I designed my extruder around a gt2 pulley with glued on 3d printed plastic parts to adapt some random sized gear I had laying around. Besides the plastic used, I have nearly $0 invested. However the next time around I am planing on purchasing parts and building two identical extruders. I plan on going the belt route. However I am going with a 40T pulley and either a 20T or 16T small pulley for the motor. At that point I will purchase whatever sized belt is available or fits my needs. I do not need more than a 3:1 ratio and 2:1 is nearly perfect.