2 vintage brands to look for are Griswold and Wagner. Griswold is probably most popular with collectors. The old cast iron skillets were polished finer than new ones and may be a bit thinner. I've been told the fine versus rough finish isn't a big deal but can't tell you for sure. Personally I prefer the fine polish of the old ones. If nothing else at least they cared enough to do it. You're not likely to see much cast iron in the thrifts these days. It does turn up in garage and estate sales occasionally and there's usually at least one vendor at any antique mall that specializes in it. More common skillets and such shouldn't be too painful pricewise.
There are plenty of how-tos for seasoning cast iron on the internet. A well seasoned cast iron pan cooks as well or better than a good non-stick pan. A look at past ebay auctions should give an idea of prices. I haven't bought any in years and can't say what a fair price is.
Rodney
Edit due to fat fingered typist.