Check out garage sales, flea markets, thrift shops, etc... and DON'T pay too much. Eventually you'll find some better-than-average cables for peanuts. I'm using some fancy Japanese-made six-nines-OFC-plus-silver cables I bought for, well, peanuts. They seem to work a little better than the 10/12 gauge zip/lamp cord type stuff I used to use.
At normal lengths, speaker cables have probably the least impact on sound (other than perhaps power cables); interconnects are more important. Connectors can matter, though; bare wire connections are usually best, but you have to be careful to create good connections and not have any shorts.