My biggest problem with RK is the inconsistency of service. NEVER EVER place an order with in in stock items and pre-order items... They often mess up orders and sometimes don't reimburse postage, despite their mess up.
I do hate the way the main page of an item says Available now, but the item is actually a pre-order. That's what I hate the most. PLus they don't have a postage calculator so you can't see how much the postage is until after they email you.
Having said that, the last couple of orders I have done with them have been pretty smooth.
BBTS is a good one I think, but postage can sometimes be high. And they will put prices up on a shortpacked item (as do RK)
Ebay, is well ebay. you take your chances there. I have never used TF source as I have little interest in 3rd party stuff and their postage seems very high.