Yes you can buy by the pound ($40 for in stock, $45 for anything needing to be made).
Best bet is to take a list with items # of what you're after.
Absolutely this. It's worth going in knowing that while they make our games' miniatures, they don't play the game. So if you go in and say you need a RAC Thunderbolt, they may not have the foggiest clue what you're talking about. Item numbers, though? THAT they can find without any trouble. Go by those instead of unit names wherever you can- ESPECIALLY when it comes to spare parts. You may know that you need an ATM-3 for an Uller E, but Melissa will likely stare at you like you're from Mars if you say that.
(Also, I'm not saying they can be bribed with alcohol, but it can't HURT the cause. Just sayin'.)