I've always collected 6th scale figures and never considered going down the Mezco road until I found this figure today; yep, I didn't know it was coming but it was a must-have even though I had to pay a bit more on the secondary market. I was thinking about doing the same for the white-suited version but think I actually like this one better. Not being familiar with Mezco, was wondering if they do PO on upcoming figures?
Quick refresher: there's regular releases, PX releases, MDX (Mezco Direct Exclusives), and Con exclusives, availability varies on those.
Regular and PX releases you shouldn't have any issue finding and are pretty well available on preorder, if you don't mind waiting you'll probably find a deal on those eventually after release too.
All the other exclusives are all over the place for value rarity. MDX exclusives are only through their web site and sell out in 30-60 minutes lately. You can now buy only 2 of each, you used to be able to buy up to 6 of each which was nuts.
Some Con exclusives, (SDCC/NYCC/Random CC) are available online too and are of similar sell out speeds. The SDCC releases had a large volume so they were easier to get overall.
There's some other Con releases, Toy Fair, although I guess that's not really a Con, for instance are super rare since they're just given to journalists and are like in the few hundreds or less in availability.
Dorklair maintains a checklist and unit counts when available of releases:
https://www.retrozap.com/dorklair/#checklists
Bottom line: if it's an exclusive just buy it and don't think about it too long. Their web site can be hell to get through and check out when something popular goes up. Also they're exclusives almost always go up on Wednesdays at 1PM EST. Not every week of course but if it's a few weeks out from SDCC or NYCC I'd keep an eye out every week at those times. If I missed anything please let me know!