Well, the PDF's I've mostly used came from the Microsoft Press books. They happened to be on the CD that came with the Paperback books. I prefer reading the PDF to reading the book (also, it's a lot heavier carrying the paperbacks). Other than that I have some PDF's from Borders. There are others, but usually it depends on the author/distributor as to what formats they allow content to be released in.

I've been passively googling it for a few hours, and I am actually incredibly surprised how hard it is to find legal ways to download pdf ebooks. I think 9/10ths of the results I get are ways to download pdf ebooks illegally.

Reminds me of the 40 year old virgin. "Why don't I just give you the money... and you give me the shoes?"