What kind of Potentiometer do I need?

If you're going for cheap, you can consider a chinese nano board.
They are cheap, small and offer enough for your LED bars.
Shipping is often included in the ridiculous low prices but you might have to wait for 2 - 4 weeks for them to arrive.

As there is no financial contribution to the Arduino community in these low prices, you might consider donating a bit some time in the future as a thank you to it.