Where you can download the game will depend on where you bought it -- I presume that since you say "very early on" that will mean you bought it through either BMT Micro, Desura or possibly IndieGala (if you bought it off the Townsgame.com website then that should have been handled through BMT Micro, unless you're going waaaaaaay back to the very first time the game went on sale).
Either way, you would have got a receipt email when you made the purchase, and that will have the link through which you can download your copy. Some systems might have provided the receipt and the DL link in separate emails; but the subject lines of those emails should make it clear which is which.
Of course, keeping track of those emails from years ago isn't always possible... fingers crossed you still have access to them; but if not then you can try sending an email to email@example.com
explaining your situation; the Dev is able to track down the purchase from his end with some basic info e.g. when you made the purchase and what method you used.
The fact that you've used the term "early access" hints that you're familiar with Steam, I presume you didn't get your copy through that platform but you should be entitled to a Steam key as part of the alpha access deal. That would be something else to bring up in your email to the Dev; because it makes accessing the game really simple for the future. If you download the game through Steam, you can copy the files straight out of the Steam folders to keep a backup for yourself and it works independent of Steam... in other words, the Steam version of the game is just the normal version + detection for the Steam overlay, so you can run it as though Steam wasn't there.
Hopefully you can get access to your copy swiftly and get back to playing the game ASAP
What's that you're eating? A nice, juicy apple? You weren't supposed to eat that you fool, you were supposed to make it into a pie! - last words recorded words of Francis D'Avre before he went looking for snowcherries, but found a hungry Yeti instead.