@Tester (one of the experts who post here regularly) recently posted a good overview diagram of how evap systems are configured. I hope you can see it OP by clicking on the link below.
Taking a look at Tester’s diagram, you can see there’s lots of places for an evap leak to occur. The purge valve is a common one, it gets stuck on sometimes, but it sounds like you’re sure you’ve eliminated that as the source of the leak. So you now have to just go through all the other possibilities one by one. If you could find it like you’d find a leak in a tire – putting it underwater and looking for bubbles – that would definitely make it simpler, but alas, you can’t do it that way with a car. Shops have a sort of similar way as the under-water method to find an evap leak fast, a machine that injects smoke into the top of the gas tank. Then they just look around for where the smoke is getting out. If no smoke is getting out, but the computer says there is a leak, then the next suspect is the pressure (or vacuum) sensor may be bad. So one idea, ask a shop to do a smoke test. They’ll tell you what they find, then you can decide if you want to fix it yourself or let them do it.