How good are you with electronics? If you're familiar with soldering you can easily build your own with a kit such as this one. It will outperform preamps costing much more. It's built around a good opamp, and I've had great results with it. It doesn't come with its own power supply, so you'll either have to use two 9-volt batteries, two ac wall adapters (preferably regulated ones), or ideally a regulated power supply kit (also sold on the same website).
Another advantage with such a kit, and one I really liked, is that for b&o turntables, you can put the circuit inside a nice looking aluminum enclosure, and put DIN plugs on it so you can plug the turntable in directly without any adapters. I'll dig mine out and take a picture.
If you don't want to build your own, I'd recommend a pretty inexpensive one. You likely won't get much better sound as you go up in price. The sound may be a tad 'different', but that's not necessarily the same as 'better'. This is a pretty common one in the US.
Art DJPre-II Riaa Premap
Although I'd recommend an inexpensive one, I'd still go with a basic brand though. On that cheap black box one you don't know anything about the circuit or power supply. You don't necessarily know for a branded one either, but I'd bet it's more likely to be designed correctly.
Austin