The only paper I've ever used was the tagboard from blick. $8.99 for 100 12x18" sheets, up to 24x36". Thick stuff - 150lb. Sometimes, if I have a coupon, I will pick up some of the more interesting papers that come in pads and rip them out. This is the cheapest way I've found to get thick black paper that doesn't look like shit or is construction paper.
I'm getting some French paper for free from someone off of craigslist today, and looking forward to using it but that Dick Blick stuff is really great.
Edwardo, thanks for that info. Checked it out and that stuff seems like a great buy. I think ima have to give it a try.