where can I buy bullet strap belt?

[ironpurple]

weapon merchants are selling Groin Guard, Lifting Belt all the time

[Altos]

Like all accessories, it has a chance of randomly appearing in the inventories of most merchants in the game, at least to my knowledge.
Try Core City if you are really getting that unlucky; the merchant district naturally has the most... well... merchants, and thus gives you the highest possible chance of seeing it without having to cross half of the game map.