I like Frame Destination. Reasonable prices, and they do a truly excellent job of packaging orders so that they're safe from even the most beligerant of delivery truck drivers.
They have Bainbridge Artcare 4Ply Alphamat in a variety of colors including several whites. The also carry the 8ply Alpharag in a more limited selection. They sell uncut, as well as cut sheets in a variety of standard sizes or you can specify custom sizing. I don't know how the price compares to what you used to pay, but they do offer quantity discounts if you're buying in bulk.