Usually pretty good supply of used skis on Craigslist and if you're eating a gnar bar, you can head over to TGR and shop their used gear forum.
That said, you would intuitively stick to a "system" ski that includes a binding on a sliding rail, then you are not going to have to deal with a remount should your BSL not match the mount on a flat mounted used set. That said, many system skis are more multi-purpose intermediate skis (with some notable exceptions) and if you're athletic and want to ski bigger terrain as you improve, you may not be thrilled with that choice over time.
Another option is to hit any of the CanSki's at Whistler or the Squamish Clearance Centre. They have a pretty big stock of ex-rentals, some of which are not heavily used, for between $250 and $350. These are all mounted the demo/rental bindings on a sliding plate, so can be set up and centred for any boot size. And typically they come with a fresh grind and wax. Less of a risk. Squamish location had a good stock of mid-fat to fat Salomon's in longer lengths (185cm+).