I personal use 21G 1 1/2" and 23G 1", I don't see the point in going smaller in dia but the length also should not go less than 1" IMO.
Where to get them? Just walk into the pharmacy and ask for them!
They are not illegal.
I just picked up 2 x 100 boxes and immediately the lady behind the counter asked "are they for steroid injection?" Why lie, I told her "yes".
"Do you need swabs also?" she asked.
These people are not stupid and at the sametime in general are not judgmental.
I'm not sure about the US (regarding the law) but if they ask what they are for, tell them you are into S&M and they are to pierce your scrotum,