If you've built circuits before, you can make your own external one. It's a very simple circuit. Go to National Semiconductor's website, and look at the data sheet for the LM833 chip. They give the the actual riaa preamp circuit. They also have a schematic for a two-stage riaa preamp although slightly more complicated, but that should be even better than the original b&o one. For the spot on the circuit where it asks for plus and minus power to the chip you can just use two nine volt batteries (one for each polarity). And since you're making the circuit yourself, you can put the DIN connectors on rather than the usual RCA. In the end you might still spend about as much as you would pay for a simple external one (about $50 if you pay for a nice enclosure, less otherwise), but sometimes it's fun to make stuff yourself, and you'll get to have DIN plugs on it too.
Austin