Deep Surplus is a fantastic source for an encyclopedic array of cables.
For work I buy all of our networking patch cables, USB cables, etc. for 10% of the cost of buying them at Staples, Microcenter, or Best Buy. I recently bought some rather hard to find white, two-lead speaker wire, which elsewhere was as pricey as $80, for $12 for a 25-foot length. I also bought a 6-foot mini (iPod) to dual RCA (for my older audiophile amplifier) cable for $2.75, compared to $24.95 at the Apple store.
I rely on them whenever I need essential cables affordably.