At $300....used and off ebay are really your only options.
Fortunetly there are some very good ones. the Aformentioned Le1700 is very nice tablet. If you want something alittle newer and more Laptop-like, I'd recommned the HP 2730p or 2740p. (Elitebook series) They are covertable Tablet PCs...so you can swivel the screen around and use it as a Tablet...or just keep it as is and use it like a laptop.
Most of the ones on Ebay were for Company use and once the lease's ended...for cheap e-sale they went.
The 2730p is Core2Duo and ranges in the $200-$300 range. The 2740p is the newer 1st gen Core i5 model. Its More expensive....most ebay auctions for it are $400-$500...but it has a better processor.
The thing thats nice about the HP elitebook series is that it has a Boe Hydis screen (think of it as an early competitor/alternative to IPS) which is a very nice screen. Comparativly...alot of the other Tablet PCs up for ebay sale, like the le1700, Le1600, Ls800, x61, TM2, TX2.....etc...most of those auctions are for the crap screens. At least with the HP, your garanteed a good screen.
At $300, there's probably not alot of options. You can get a cheap'ish Samsung Galaxy Tab, but it won't have a Wacom digitizer. For that, you would need to step up to the Galaxy Note 10.1 for roughly $500. Maybe wait a few months and the Note 10.1 may come down in price. Another option would be a used iPad 2 and buying a pressure-sensitive stylus (not sure how well these compare to Wacom).
A refurb laptop may not be a bad idea. With Win8 laptops coming out now, Win7 machines should be coming down.