How many conductors do you need?

Try Grainger

If you only need two conductors you can use the stuff from the home centers that are sold for low voltage lights. If you need more, you can get direct burial phone wiring (don't see it at Home Depot, but it's available online and probably from telecom suppliers in your area).

I only need 2 conductors.

This is low voltage DC?

Yes

The "landscape" cable will probably do you then. It's a bit heavier (the smallest stuff is like 16g) than you need but should work fine. Both Lowe's and Home Depot has it in the lengths you need.