I can't see close enough in the images from the link you provided, but it does look like they give you the polarity for the power input on the back of the unit. In that case it would be very easy to get a universal 12 volt power supply and plug to fit your unit. It won't hurt to get a unit with more amperage than you think you need.
You can get a universal power supply like this:
Enercell 12V/1500mA AC Adapter : AC Adapters | RadioShack.com
Or go bigger with something like this:
Amazon.com: Pyramid PS3KX 3-Amp 12-Volt Power Supply: Car Electronics
You'll just need to determine what size plug you need.
Some universal adapters will come with different tips to use on the end. That would allow you to choose the correct tip to fit. The critical thing here is to be sure the new adapter has the same polarity and output voltage. I can't see in the picture well enough, but the preamp has it on the back: It tells you whether positive is in the middle or on the outside or the connection and you need to match that with the new cord. From the pic, it looks like you need 12VDC and some adapters allow you to select output voltage too.
Now, about the amp rating. Actually, the electronic device "draws" amps from the supply. As long as the supply can handle the amp demand, then you are set. Another way to put this is, you should be able to use a power supply rated at a higher amps than needed, but not a supply rated at a lower amps than needed. So like Major said, you won't hurt it with higher amp supply than you think you need.
I really think the enercell from Radioshack that Majorloser linked you to would work. It is rated at 12V and 1500mA (or 1.5A) I also think that you must buy the universal enercell, then choose the tip to fit and match polarity on the preamp. If you have a Radioshack nearby, I would go there first. The employee at Radioshack "should" be able to help, but it's probably a 50/50 shot on that.
Hope that helps.
