Ask the seller for one. They might have more incentive than you to have it on hand.
This won't work for most buyers - but I could pull this off most of the time. If I was "probably picking up Tuesday" i'd be nearly 100% sure I'm getting it - as long as it hasn't been wrecked or have more rust than I anticipate from pictures/discussion. I'd say something like....I'll buy it if it has no more rust than I expect and hasn't been in an accident - could you get a carfax report and I'll reimburse you for it if it hasn't been wrecked or lived in a rust state?" Tell them exactly why you want it. I've bought like 50 subarus and helped people buy like a couple dozen others, so i've got a lot of experience. I also buy almost ever car I go look at - I don't have time to window shop and screw around, so I'm pretty serious and people get that when I talk to them. I'm up front, not going to lowball (I usually am already looking for lower priced vehicles and I'm willing to throw cash and even add more $$$ to their asking price sometimes if I want), and that makes a difference in how I communicate. Anyway - I'm just saying don't expect them to bite but I'd be able to get them to agree most of the time just based on my confidence and doing this many times. I know what people do and don't want to hear.
Also google the VIN number and see if it's registers under anywhere strange - auctions, etc.
I also concur with GD's response - if you don't find a way to get a carfax quick, just pay for it. You're time can't be that invaluable if you're buying a 5 digit car. If you're unsure about buying the car then buy one of those "packages" that allows you to VIN search a few vehicles. If you're fairly certain you're going to buy the car - then pay for the carfax. I've used carfax lots of times. Some times I do, sometimes I don't it just depends what i'm buying, from who, where, price point, etc. For most people though - just buy it. Looking at a 5 digit car and commensurate maintenance over it's lifetime and trying to save pocket change isn't really worth the time.