I know you said you'd rather buy from the local hardware store, etc.... BUT
kayakers have been trying to get around buying minicell foam for 20 years (as we are generally a 'thrifty' lot) but there's really no alternative - harvesting from other equipment (sleeping pads, etc.) not withstanding you might be best to just break down and order the proper quantity
try the web's best resource for kayak outfitting
unfortunately the shipping's what will get you with gas prices nowadays
really there's no substitute for a well outfitted boat especially when you're starting out - I don't think anyone's mentioned a piece of dragonskin or a shaper from the local hardware goes a long way towards getting the right shape for your hip pads. Usually if you budget $40 or $50 and a couple of hours of time you can have super bomber outfitting. I find if I cut a special piece for LL playboats to go under the stock hip pad that leaves a gap for the back band connector but goes into the gap all the way to the plastic it really tightens things up
best of luck!