Jason and Mr Kitts used injectible methyl-B12 and they always swore by it. However, they had a prescription and had it made for them at a compounding pharmacy, so it was a sterile solution. I would think that any compounding pharmacy could make it for you; check your local phone book for compounding pharmacies.
I honestly don't know how they make sterile solutions for injections, but I'm going to assume that it's not just crushing up pills into a powder and adding it to distilled water. There's probably something about making sure that the crushed grains are fine enough not to cause problems if they remain suspended and something else about making sure that the solution remains sterile (bacteria-free) and safe for injection. So if you're planning on going this route, I would recommend talking with your vet about a prescription for injectible methyl-B12, so that you can be assured of the quality and safety issues.
Miscellaneous unsolicited comments:
Remember that B-12 (at least in the liquid form) is light-sensitive. Keep the bottle in a dark place. If you inject the B-12 into the bag of liquids (instead of injecting it into the port on the line), you'll need to keep the liquids away from light as well.
Is it definitely methylcobalamin you want? Because a lot of cats get B-12 supplementation with regular B-12 (cyanacobalamin), which *is* available in injectible form. And cyanocobalamin is what most vets add to sub-Q fluids when extra B-12 is needed. (Methylcobalamin use is pretty rare.) You can get injectible cyanocobalamin from your vet, via special order at some pharmacies (with a prescription), or through some local feed stores (without a prescription). Also, you don't need to add it to sub-Q fluids; you can do sub-Q injections just like you do with insulin (we even use our insulin syringes for our B-12 injections).
Remember that methyl-B12 alone is not the cure for neuropathy. Diabetic neuropathy is caused by unregulated diabetes and the cure for DN in cats is regulating the diabetes. If unregulated, methyl-B12 can help slow the progression of DN; and, if regulated, methyl-12 can help speed the healing; but methyl-B12 alone is *not* the cure.
Are you monitoring BG levels? Because (a) if you have neuropathy, you'll want to keep an eye on blood sugar levels and (b) subcutaneous fluids often lower the amount of insulin needed. So if you've been increasing sub-Q fluids without monitoring BG levels, you may be giving the wrong amount of insulin.