It really depends on the genre you're looking for but a cheap way to introduce yourself to some quality B movies is by buying some of those 50 Movie Packs from Treeline. They've got around 15-20 box sets with 50 films in each. I own around 11 and there are quite a few gems.
3 that top my list of side splitting laughter (although they were meant to be serious):
1. Robot Monster - the bubbles in the cave are classic as well as the line "my name is Ro-Man from the planet........Ro-Man. 2. Plan 9 from Outer Space - Ed Wood's best 3. Bride of the Monster - the fight with the giant squid and the actor shaking it to make it look like it was alive is true b-classic.
^ Though I got the impression the OP might want B movies from the 40's and earlier, I actually don't know if I have any on DVD so can't recommend any for sure. Thing is, IMO, a large number of the "A" movies from that era, or what I think are intended to be A movies using "name" actors, really are B movies by any reasonable quality judgment. They really cranked them out, and budgets were often B movie low too, especially in the 30's and during WWII.
Just throwing something out there to show what I mean, I think the vast majority of the Abbott & Costello flicks are B's, and most in the WB Marx Brothers Collection box too. Patrick mentioned Gun Crazy: I would consider that whole Film Noir Classic Collection box as B's or at least borderline. Which doesn't detract from their entertainment value.
So...I don't know what really qualifies as a B movie from the studio system era. Generally I think it's just low budget and production values. They are still working on getting many of the more famous "classic" A movies on DVD, long way to go. I guess, like somebody mentioned, a cheap box set of PD movies would have a lot of B's.
To me they seem *like* B movies. To others they don't, and I'm certain they weren't intended to be B movies. But really, watching many (IMO most) of the ones in those sets, and comparing to the best of what the characters did... I suppose many would say when famous actors are in a movie that they can't be B movies, but I look at the scripts and production values. Like sticking a famous "brand" on an inferior product, expecting people to buy the brand (actor) regardless. Weekly entertainment fodder. To call them top quality is impossible for me, compared to other stuff of the same era. Considering the system of the time, I don't think a B movie of that era could be judged the same as one of the later era, which are a little more obvious to pick out I think.
So: what *would* be a B movie in the OP's category? I dunno...the shorts?
Stuff like A&C MEET FRANKENSTEIN is most certainly a "B" movie. The budget was the next to lowest for Universal that year and the budget was lower than "B" stuff like HOUSE OF FRANK and HOUSE OF DRACULA and other horror films of that decade.
Even the Poe/Corman pics had a higher budget than what we could call "B" flicks. Those films certainly aren't on the same level as other Corman flicks like CREATURE FROM THE HAUNED SEA, THE WASP WOMAN and THE LITTLE SHOP OF HORRORS.
Nikki, if you're wanting to most for your money then check out this site:
Click on the 50-movie packs and you can see what's on each set. They've got at least three sets of horror titles, three of sci-fi, a musical set and another set with mostly noirs. The HORROR collection is a good place to start since it's got all sorts of Lugosi, Karloff and other films like that.
You'd be better off buying them from another site though. You can find them at places like DigitalEyes between $13 and $18. You can also go to Best Buy who stock several for around $20.