Yeah, the easiest way to find good fiberglass is usually some sort of farm supply place, where they'll have it as electric fence posts. 3/8" or 1/2" rods work best for most blue weapons (though 3/8" can only go so long... 30" or so?).
You don't say where you're from so it's hard to say, but the
Tractor Supply Company seems to be one of the most widely spread chains out there that I know sells fiberglass fenceposts - Fleet Farms and Farm & Fleets also sell them if either of those are near you.
You can also use PVC pipe, which is available everywhere (incl. home depot). It's heavier than fiberglass, depending on how you use it, and is more likely to break, but will work just fine. For a blue weapon you'll want 1/2" or 3/4" sch40 (3/4" for long weapons) or 1/2" sch80 PVC. Schedule 80 PVC has thicker walls than schedule 40, making it heavier but stiffer and more durable. You can find it at Home Depot in the electrical section as Conduit (which also comes in sch40, you'll have to look at the tag or the pipe itself to be sure). PVC is also dirt cheap.
As Peregrine said, the default "camping pad" comes from wal-mart, is flat (not egg-shell style), blue, and about 3/8" thick. It sells for a bit under $6 a roll, and is enough for a number of swords. The stuff at Target in particular is usually worthless for our weapons - it's too soft and doesn't recover quickly enough.
It might also be helpful to you to check out the weapon construction section of the
Belegarth Wiki - it includes almost all of the angelfire guides, as well as articles on all sorts of
materials - be sure to check the subcategories.
Hope that helps!