Craigslist and Facebook Marketplace seem to be the best options since they tend to be more local oriented, and shipping can be a big expense on something the size of a lathe or mill, plus you really want to be able to look over a used machine. It can't hurt to watch Ebay, it is always possible you will see something close enough to make the drive.
As far as machines the main ones that fit your size are the Clausing 8520/25/30/35 and Rockwell 21-100, both around 700lbs and typically with an asking price of $1500-2500. A little bigger would be a Burke or Powermatic Millrite which runs around 1100-1200lbs. I think Enco at onetime sold a knee mill similar in size to the Millrite. There were also some small Taiwanese knee mills made in the 80s that would also probably work for you, but I don't know the brands or model numbers. I've seen two of these in person and they were completely different, one looked like a 1/2 scale Bridgeport, the other was very similar in appearance to the Rockwell mill. Parts and manuals might be an issue but they looked like nice little mills.
Other than the Harbor Freight 6x26, Grizzly also sells a similar mill that seems to have better specs, as well as two slightly larger 8x30 versions.
Grizzly G0729 6x26" mill
Grizzly G0678 8x30 mill
Grizzly G0695 8x30 mill
Last option is the Precision Matthews PM-835S which well over your weight at 1400lbs but still quite a bit lighter and compact than a full size Bridgeport. It is quite tall though.
PM835S 8x35 mill
New all of these mills are over your $3000 budget, but used would probably be at a good price for you. The Grizzly mills have been around for quite a while and similar ones have been sold by Enco and others, so they do turn up used.
I have a Clausing 8520 which I am quite happy with. I don't have any personal experience with the others but these are the mills that were on my list to watch for (except the 835 which was way to tall for my shop) when I was shopping for a mill. I do know a couple people who have Millrites and they like theirs.
There was a member here who bought one of the Harbor Fright 6x26 mills last year if you want an actual owners review.
Talk me out of a used Harbor Freight mill