Where to buy catheters: first, check with your insurance company because if your supplies are covered, they may have a preferred distributor, like Byram Healthcare. Medicare does cover continent ostomy supplies and so does UHC — I'm sure many others do as well.
Second, your surgeon would tell you what size catheter you need. Typical sizes are 28FR and 30FR.
Marlen and Medina are popular brands of catheter, 15 inches in length or longer.
Direct suppliers for catheters and other stuff: just Google "continent ostomy supplies." My favorite company is Austin Medical, where they make Ampatches to cover your stoma and are generally very helpful if you're new to the continent ostomy world: https://ampatch.com/