I can't tell you where you can definitely find parts, but I'd suggest giving Florida Rigging and Hydraulics a call. 561-863-7444. Talk to Tim. He's the Lewmar guru.
I understand your problem. I have an old Lewmar 3000 that is in the same boat, if you'll pardon the expression.
For what it's worth, I recently had trouble with the unit, and tore it completely apart. What I found was that the bearing directly below the capstan-gypsy was an open cage, normal steel bearing, with a chintzy seal covering it. Ultimately, it turned out that the bearing was seizing up because it was rusted. I had to press the bearing off using a 20 ton press because the common steel bearing had rusted to the stainless steel shaft. Even then, it took a little heat from an oxy-acetylene torch to get things moving.
I mention all of this because I thought the gearbox was trashed on my unit, too. It may just be that bearing that's hiding under the seal on your unit, too. The good news is that you can get sealed, S/S replacement bearings at a good bearing shop. They're pricey, but they'll last forever. Or you can do what I did and buy a standard sealed, pre-lubed bearing, slather it down with anti-seize and add it to the list of to-do things for the yearly haul out.
In the event that doesn't work, and Fl. Rigging can't help, you might try Sailorman in Fort Lauderdale. They always have a good selection of used windlasses. Here's the link: Sailorman New & Used Marine: The World's Largest and Most Unique New & Used Marine Emporium
Finally, if you end up having to replace the unit because you can't find parts, be advised that Vetus now owns Maxell, and their customer service-warranty repairs, etc., has gone in the toilet. Not my opinion, but the opinion of one of the local boatyard managers I was talking to a couple of weeks ago.
Lewmar, as you've probably found out, is bad about changing models, and not stocking parts for older units. Since Maxell is now owned by Vetus, I personally wouldn't touch their stuff any more. That sort of leaves Lofrans and Quick. I installed a Quick unit for a friend a few years back, and to the best of my knowledge, he hasn't had any problems with it.
Good luck, and don't be afraid to tear your windlass apart. Just tell yourself: "If it's already busted, I can't bust it any worse by tearing it apart." Maybe you'll be lucky and just find a rusted English bearing like I did.