Walmart has several types of Auto Drive Car Stereo Speaker Wire on sale:
14ga 50′ for $3.17 Brickseek.
16ga 50′ for $2.01 Brickseek.
18ga 100′ for $2.55 Brickseek.
Select free store pickup, otherwise shipping is free w/ orders $35+.
In store prices are a lot higher around me: 14ga and 18a are $12.97, while 16ga is $10.97. I was able to do online order at lower price with store pickup.