Starting with stuff you can get via completely official and legal means (everything here is also available from Netflix, with some of the Wrestling Gold family of stuff available via "Watch Instantly" streaming):
Pick up some Memphis. The TV formatting and wrestling style (lots of great brawls and epic main events) will be easy to get into if you're used to this decade's WWE stuff. Maybe start by picking up the Wrestling Gold DVD set at retail to get started as it's cheap and has a good amount of quality Memphis footage, with alternate commentary by Jim Cornette & Dave Meltzer to give all sorts of fun historical info. Some is edited but it's a good start. I was hooked by seeing some of the same matches on old VHS tapes from the same company. The 2nd box from the same company has 2 Smokey Mountain Wrestling DVDs and some OVW, which is newer ('90s and '00s respectively) wrestling booked by Jim Cornette to have a similar feel to Memphis and Mid-South (and these also have Cornette/Meltzer alternate commentary tracks).
Speaking of Mid-South, they have 2 official retail releases available that have some good stuff:
The Watts family sells a lot of Mid-South footage at http://www.universalwrestling.com but they have no compilations besides the retail ones, just a lot of full TV episodes and house show footage, so it's not the best place to start.
Oh, and back to Memphis for a second. There's the Andy Kaufman "I'm From Hollywood" documentary that covers the Lawler feud. Though the timeline is messed up at times and they skip a lot, it's a blast to watch. Netflix has the out of print version with "My Breakfast With Blassie" on the B side of the DVD.
There's also a solid "Best of the Von Erichs" DVD that's out of print. (All of the commentary has been removed, though)
Of course, there are also the various WWE releases of older footage. The Flair sets, Pillman set, Bloodbath set, '80s stars set, and Andre the Giant re-release are a good start.
Now, for the less official means of procuring footage.
Memphis is an odd duck. The masters are spread out all over the place and the actual owners don't seem to care about others doing anything with it. There's a lot available from your regular burnt DVD traders/sellers. If you want something a little more structured:
Highspots has some releases they've put together that are endorsed by Cory Maclin of the current Memphis wrestling. They're pressed dual layer DVDs (not burnt) and the video quality varies, but there is some good stuff on them. So far, they've released:
Classic Memphis Wrestling - 70s Arena Footage
Classic Memphis Wrestling - Jerry Lawler vs Andy Kaufman
Classic Memphis Wrestling - Strangest Matches and Wildest Brawls
For the DVDVR '80s polls, Goodhelmet has put together a big set for the voting process. Full of great stuff and while it's recorded in LP mode to minimize the the number of discs, most of the stuff holds up ok. Enough people are participating that you know you're not being ripped off or anything.
Rick at 70s-TV.com has a bunch of stuff, largely consisting of compilations that he's put together. The matches are clipped at times but even when they are, his discs and sets are great overviews. He's sold stuff on eBay for years and is very reliable.
Jerry Lawler's website sells older versions of Rick's comps as well as DVD conversions of some of his old home videos.