Probably very near by wherever you will be based if you join the army...if there's a shop near a regiment and know they can make sales if they stock that particular regiments stable belt, you can be sure they will Worrying about a belt is for:
A. As soon as you're ordered to get it...
B. Soon to passing out of Phase 1 (if needed for the passout parade) or nearing the end of Phase 2 in preparation of moving to unit.
either way a bit soon seeing as you will have no use for it starting training, just like the beret, you will not be allowed to wear one yet. It is earned.