It depends where you are in the world, but AB Universe, Tykables, and Rearz are generally the 3 main ABDL diaper companies to pick from. You can order direct from their websites, or order from a reseller (like Daynite Care if you're in Canada).
Since this is your first order, you could just order a sample of a few instead of a whole pack?
All three of them make high-capacity diapers that can get thick, all of them ship discreetly, and all of them have cute and fun designs as well as plain versions of the same diaper if you're not into that.