If you're looking for old or out-of-print (OOP) charts, you have a few options.
One, of course, is eBay or other auction web sites, to see if someone has put the chart up for auction.
Another is The Old Stitch, a small company specializing in gently used cross stitch charts/books.
Adams Used Books is a similar service.
I haven't used any of these services personally, so I have no opinion about them one way or the other. But if you're desperate for a chart - and it's happened to all of us - then these might be worth checking out. Of course, you can also ask around - your LNS, your fellow stitchers, online forums, garage/yard sales, etc. can all be potential sources of a lead on a difficult-to-find chart.
Good luck and happy hunting!