I acquired some U-100s bought from Walmart, but they aren't ideal for using with a U-40 insulin -- they aren't marked for half units and the syringes are set up to deliver up to 30 units, so the markings are close enough that they're really suitable only for dosing 1 unit at a time, not fractions
you can get U-100s online which are much easier to use (read, prepare to inject smaller doses) many experienced members can recommend their favorites
you can buy U-40s (look for the "small dose" .3 syringes) from several of the vendors online -- check the shipping costs too -- the most common brand is VetOne, but there are others, close to the same price range though, vendors I remember were something like PetMeds and PetRx .. not as cheap as the Walmart offerings, but precision can beat price when you work with family members and with droplets
look at all the demonstrations/videos of injecting -- bonus if his skin is loose enough to pull up a handful of skin or pull up only on the fur (over the flanks or either side of the backbone), I've tried several, if you think about sliding the needle in smoothly though skin stretched in tension, it seems to work well, neither really fast nor hesitantly slow
Catcat is always hungry so I shoot while he's eating, though he hates ear pricks and does growl, hiss, swat, try to bite (thank goodness for Neosporin, my scars heal quickly) when I test -- some of it, I think, is my own mental attitude of "look, cat, we gotta do this, it's important for your health" -- tough for me because I have a lifelong needle phobia which no therapy nor treatment has been able to eradicate
