If you're looking to buy a full system (complete with keyboard / mouse / monitor), then I'd suggest someone like Novatech or Scan. I've ordered parts from them before and they seem pretty solid; Novatech staff are also really helpful and very friendly without being pushy either. They are also large companies so availability of choice and parts should be very high and that means price should fall into your price range :)
If you're just after a base unit, (no monitor, mouse, keyboard - handy if you have these parts already), then I'd suggest either of the two companies listed above, or the place where I got mine - I've posted about them before so I won't fill up this thread, but I have no complaints from service received from any of these three.
I've heard Ebuyer may be a viable option but I've never used them before.
Hope it helps.