Inet usually has these available. If they're out and you need them fast, I have a spare couple of the fat double alphas I'd be willing to part with. Let me know.
I've got 3 or 4 thick doubles, from Star Trek SE's which I move dinto thin doubles. I'd send 'em for a small fee to cover shipping, where you located, you don't have that filled in.
I'm just wondering who's covers you decided to go with for your ST replacements. I'm quite fond of Andrew Parmet's work on the series. They basically feature the poster cover art and have nice custom backs.
I used the covers that came with them. I just used a paper cutter to shave a little off, which ever side looked like I would lose the least material. Yes, basically I panned and scanned my covers.