I've used 22g, 23g, and 25g to inject. The only difference in pain I've noticed is at the time of injection, the smaller needles tend to hurt less going in, but the discomfort in the days following is comparable once your technique is good. The length doesn't really matter as you can control how much of the length you use. I can't imagine anyone who doesn't have a ton of extra body fat needing a whole inch and a half for a thigh injection (although possibly for an arse one), I don't even use a whole inch and it's still reaching muscle. I'm currently using 22g, because that's what Strohecker's sent. I'd rather have 25's, but it's not worth it to me to pay for different needles.
As for where to order sharps online, I'm afraid I have no clue, hopefully some other guys here know some cost effective places for you.