Like Bigfoot said, check places like Sierra Trading Post for XXL and XXXL clothing. They quite often have some on hand. Ditto on Cabelas, too. They have extended sizes for most of their store brand and most of it is very good for the price, if not especially lightweight.

Some other comments
-Mountain Hardware tends to be fairly large sized. I think Columbia is, too. Patagonia and Marmot seem to be one half size to a full size smaller for XLT and up.
-Canadian and European clothing is usually one size smaller than US (XXL Cdn = XL US), so Arcteryx, Mammut, etc are probably out. And don't even consider Moonstone, they use Japanese sizes for most of their range.
-If all else fails, there is some nice, fairly lightweight gear in golf shops that can work for backpacking, too.

For your size, I'd be very careful about ordering from Beyond Fleece. I ordered a custom jacket for my dad (who's the person I buy the large stuff for), and it looked like their sizing algorithm goes badly off the rails for people over 250 pounds. They also lied repeatedly about delivery dates, or were woefully incompetent; and trying to return the totally wrongly cut garment was the worst customer service nightmare I had in the last 20 years. Since I've heard very good things about them in the past, I won't post my correspondence here. I'd definitely call them and get some written/email assurances if you consider them.

Oh, and do get some trekking poles.