Keep your eyes peeled on Craigslist, but use Searchtempest to look at all of he CL's in your drivable area. Free is not necessarily convenient and you need to figure what driving say 5 hours each way is worth to you for a "free" or low priced table. You also need to take the condition into account. A $100 table you can get in town may be a better bargain that a free table that requires a 10 hour round trip drive.
I tend to try and stack things though. See if people have any other stuff they want to part with or if there is any other business that I can do out in that part of the world, should I wind up out there. From seeing old friends to stopping at interesting eateries or getting something made locally to that area etc. If I can get enough things to line up, I call it a go. I just did a 6 hour round trip to pick up a guitar amp for example, but had a bunch of stops squeezed in after the pick up, and it was a nice spring day and the drive itself was enjoyable.
If you do wind up going far and you are gong to tote back more than you can fit in your car, look into renting an enclosed trailer (assuming your vehicle has a trailer hitch). They cost less a day than renting a truck, and there are no mileage fees associated with them. The mileage fees on the trucks are upwards of a buck a mile and they get bad gas mileage. The $20 truck rental fee is noting compared to what you will pay in other fees. The trailer is roughly $20 for the day around here and you can put hundreds of miles on it.
You don't see free tables every day, but you do see them regularly enough that if you keep looking and are willing to do some driving you will find one. Also if you find a $100 one closer to home you may try offering them $20 for it. They are not exactly hot commodity items.
Good luck and happy hunting...
Also, check out my blog for how I moved my box table out of my house discretely. It goes over how to take one apart to the point that it looks like a desk. I also have a en entry for my gadget to help you get it up and down stairs on your own.