I'm aware that fleece isn't the best warmth for the weight, but I still like it and am planning to make a lot clothes for my kids out of it. I say "a lot" because they have this bad habit of growing!
The most complete source of Polartec brand fabric I've found online with great prices on many items is:
Mill Direct Textiles
I haven't received my first order yet, but it was shipped the morning after ordering. Meantime I've been going over their swatch pack of over 100 samples.
They have many "all time favorites" that they keep in stock at all times- at reasonable retail prices. In addition they have lots of short rolls and closeout items at great prices- Wind Pro for $15/yd, Powerstretch for $8/yd, Powerdry for $6/yd, etc. If you need a lot of fabric you can order full rolls or 15 yard bolts for even better prices- Powerdry seconds are $3.20/yard if you order a roll.
Here's what their website says about the company:
Welcome to Mill Direct Textiles. We are a newly formed company made up entirely of former employees of the Malden Mills Retail Store. All of us are also former employees of the Mill and have a vast experience with the fabrics we are selling. We are eager to help you find the right fabrics for your needs.
We work directly with Polartec LLC in Lawrence, Massachusetts and can, therefore, offer you the full family of Polartec® fabrics—known for their quality and as the most technically advanced outerwear products in the marketplace.
I believe that at the time Polartec became a stand alone company from Malden Mills (about 2007), the factory store closed and Mill Direct was formed.
The next source I found was through a Craigslist ad. A woman who has spent the last decade making polartec garments is closing her shop and selling her remaining yardage. She had bought a trailer load of Polartec fabric in 2007 when Polartec spun off.
I'm not comfortable putting her contact info here but I will email it to anyone who wants it.
Her prices are: $15 for Powershield (black only), $8 for Wind Pro, $6 for 200 weight. You would probably be best off getting Mill Direct's swatch packs first unless you know exactly what each fabric type is.