Duchess,
The 21 ga needles are usual ones suggested, they are bigger, but the serum is kinda thick, so the bigger the needle, the faster the serum goes in, which suits nurses in a busy office better than our backsides, lol.
That is good for the nurses, but I prefer to use the smallest needle possible, so I ordered 31 ga needles in bulk, and warmed my vial in my pocket for about 15 minutes prior to drawing out the serum. It goes in slower than the big needle still, but most times I never felt anything, and going more slowly disrupts less tissue, too, which also cuts down on the sting..
My nurse suggested this procedure, and it really made a HUGE difference for me. Also, since I don't have much fat back there, the 1 1/4" 31 ga needles were perfect. All needle are NOT made equally well, so buy the highest quality ones you can find, and the points will not have any burrs on them, which makes them even less painful to use. I ordered mine from a company in Fl that specialized in diabetic supplies, I think the brand I used was BD Syringes. You may have to order the actual needle tips and syringes separately, and yes, they are the 3ml size. Google it, and compare prices...
Hope this helps..
Missy