Don't buy them from the archery shops. Make them.
You can buy 50 metres of guy rope from
http://www.towsure.com/ for £4.25. This makes approximately 175 finger slings. Use 6mm windscreen washer tubing from your local car accessory shop for the center and sliding bits. You could always stock up on tubing and buy 10M of said tube from somewhere like
http://www.aruncas.com/subcat39.htm (costs £5.86) and 10M will make ????....lots!
ps It's easy!!