Any reasonably quiet 12 supply rated at 1.5A or better should work just to try it on your desk, (motors not moving, or not driving wheels to move the 'bot).
I use a "universal laptop power supply that has a 12v setting when I am doing development work on my GiPiGo, (on the bench, not moving), and it works well. If you want mobile power, either one of Dexter Industries/Modular Robotics lithium battery packs, (recommended), or an 8-cell battery holder with GOOD batteries will also work, but the run-time with NiMH batteries will be quite limited. Alkaline batteries are even worse. I originallly got a 15-20 minute runtime with alkaline batteries, and about 45 minutes to an hour with good NiMH batteries depending on how much you use the motors. Cyclicalobsessive uses the expensive Panasonic batteries, (which I did not get), and his runtime is a bit longer. The Modular Robotics TalentCell 1203000 batteries give several hours of solid runtime before they die. Note that the Modular Robotics battery pack is rated for about 4 hours, but that will depend on how much time you spend with motors running and how effectively you manage battery power. In any event, it will be considerably longer than an 8-cell bettery pack with NiMH batteries.
If you do decide to go the “8-cell battery” route, @cyclicalobsessive has done considerable research on this subject and he uses Panasonic’s fancy, (and expensive), NiMH cells. I can personally attest to the fact that alkaline cells don’t work well, and cheap NiMH batteries are little better.
Amazon has a version of the TalantCell 1203000 you can buy.
If you are in the US or Canada, I would get them from the Modular Robotics site. Elsewhere you can try to find them on whatever shopping sites are available.
Get an 8-cell AA battery holder along with some cheap batteries and try that just to see if the motors work and the LED stops flashing. (Cheap alkaline batteries won’t last long, however all you want to do is verify that the system WORKS before investing money in GOOD batteries.) If that works, then spring for the expensive NiMH batteries or the Modular Robotics lithium battery pack.
One last note:
If you do end up going with the expensive Panasonic NiMH batteries, get a GOOD “intelligent” battery charger. The cheap chargers just dump power into the batteries without regulating the charge and that wil (literally) burn up your batteries.
Try that and let us know what happens.
low power = flashing LED. . .
That totally slipped my mind - thanks!