I just bought a very nice HDMI cable from Amazon.com for about $2 + $3 shipping. There seems to be a lot of similar cables in the $2-10 range, which beats those $20-50 cables the stores seem to stock, and it works fine and appears to be of a decent build quality.
Parts Express is another good source for cheap cables. I bought all my optical cables from them for around $2 + $5 shipping. They've been working fine for years now.