Some pool supply shops do close up over the winter, but others stay open year round because they also carry stuff for hot tubs or skiing as well.
I actually use all three ingredients in my soil mix: 5 parts peat moss, 3 parts silica sand, and 2 parts organic perlite. I buy the big bales of Premier brand peat moss from either Lowe's or Home Depot. The sand I use is discussed on my very first thread here ever:is-this-sand-okay-t19144.html
(BTW, looks like I was mistaken on the TDS reading on that sand. I wrote on that thread that it only went up to 6 ppm, so it must've been something else I was remembering that went to 15. The water standing in my trays after a heavy rain usually averages anywhere from 5 to 30 ppm.)
And the perlite I use is this brand:http://www.amazon.com/gp/offer-listing/ ... dition=new
I originally found that at a small locally owned nursery for almost half that price, but when I went back to get some more the next year, they had to special order it because they weren't going to be carrying it again that year. Might be worth calling around to some local plant nurseries (if they're still open this time of year) to see if you can find any without fertilizer added to it. (Gosh darn you, Miracle Gro company!) I'm not familiar with the therm-o-rock brand, but if Matt recommends it, I'm sure it's safe. It's always cheaper if you can find the ingredients locally since you don't have to pay for shipping that way.
I'm thinking now though that I might omit the perlite the next time I need to repot anything and just go with 50/50 peat and sand because I've been reading Peter D'Amato's book "The Savage Garden" lately and he said this about perlite:
"Commercial nurseries generally prefer to aerate the soil in potted plants with perlite rather than sand, because sand is very heavy and leads to high shipping costs. However, most perlite is also slightly alkaline (pH around 8), so never use more than 20 percent in the soil mix for acid-loving CPs."
Yikes! I have a Sarr that's been in 50/50 peat and perlite for over a year now that barely grew any at all this summer. I guess I'm going to have to go buy another meter now, to test for pH!