Howdy.
I too live in an area where most hardware stores don't carry 5/8" thick foam (it just doesn't get cold enough in most of the NW to need it).
You've got a few options. One is pool noodle, as was suggested above, but it tends to be thicker, breaks down faster and can be a pain to work with. (Never try to shave pool noodle down to be thinner, it reduces the life of the foam dramatically!)
Another option is to double up 3/8" inch stuff. Simply put the first layer on, cut another piece lengthwise (easy if the foam comes pre-split, an most does) and put it over the first one. There will probably be a gap down the length of the weapon, which you'll need to fill with another strip of foam. Occasionally you can get really lucky and find one piece of foam that has a large enough inner diameter that it will fit around the piece of foam that fits your core, but that's pretty uncommon.
Finally, you can order foam online (that's what I do). I get mine from McLendon's in Seattle (try
this one or
this one, depending on pipe thickness.) Stores that specialize in plumbing are more likely to have a wider variety of foam. You can probably find a store more local to you, try calling stores in your area so you don't waste your time driving all over town. When you call, make sure you talk to someone in plumbing and ask for 5/8"
Wall insulation (or 3/4" wall, which is more common and perfectly legal.) They'll probably ask you if you're using copper or iron pipe, and you're not using either, but depending on the core you
are using, it's probably about the same as either 1/2" or 3/4" copper pipe.
~Matt,
Weapon guy,
Marshal