Hmm . . . I'm surprised it's so uncommon. I've given cats shots quite a few times. It's super easy to give in the scruff of the neck (just gotta make sure you jab really hard to poke through the skin all the way. Some have to be given in the leg, and that makes me a little more nervous. Luckily I've only ever had to give ones that can be done on the back of the neck. Just pinch a bunch of skin, stick in the needle, jam down on the plunger.
P.S. IIRC, a full set of cat vaccines at Cal Ranch is like $7, including syringe, just to give you an idea of how much money you save by doing it yourself.