none of your standard stores will carry decent kevlar wick. some hardware stores will have pure fibreglass wick, in the form of oven gaskets, but that's not too great unless you wear a resparator when you spin.
you have to go to a specialty shop, like a juggling shop, for any decent kevlar wicking. if you're in the states,
http://www.seriousjuggling.com/Fire_Wick.htm has a pretty good selection for the price. shameless plug for the local shop, but i'm sure you could find alternatives.
also, don't get too hung up on using kevlar for your poi. my first, and honestly greatest, wicks were made from bar mop towels I got from walmart for $5. huge flames, long burns, and cathederals don't unravel as they burn up. they're only good for a few burns, but then you make another set of heads and get going.
Also, if you provide your location, chances are someone lives near you and has the low-down on the best shops for fire artists in your area.