Haven't had to get needles for my longarm yet, but when I do, if I can't get them at the store that sold the machine to me, I'll probably check King's Men quilting supply at
http://www.kmquiltingsupply.com/Needles/default.asp
I've ordered from them so far for long zippers and rulers and have been impressed with their speed. One quirky thing about them, though, is that if they don't have your item in stock, they don't backorder it. You just have to re-order it. Calling them to see if they have what you want might be a good idea.