The B12 used with GI conditions is called cyanocobalamin.
The B12 used for diabetic neuropathy is methylcobalamin.
Cyanocobalamin comes in injectable form. It is inexpensive ($7-8 in the U.S. at certain online retailers, such as KVSupply.com) although it *is* prescription.
Methylcobalamin (also referred to as methyl-b12) is often in capsule form and is available without a prescription. I always used the Zobaline brand, which is specially formulated for cats, although many use human brands (you just need to make sure there aren’t artificial sweeteners in it or other ingredients that aren’t safe for cats). It might also be available in injectable form but I’m not familiar with that.
My guess is that you’re looking for cyanocobalamin.