It's quite hard to give a good recommendation when we don't have an approximate knowledge of stores around you.
In general though, most electronics shops will usually have a significant markup on items. If you can find a general surplus store, they usually have fairly good pricing on them, if they have it available.. As long as you're getting a Cat5e cable, you're doing fine, those support speeds upto 1gbps, and that's where most motherboards max out on network speeds.