These days, it's either VintageMicros or eBay. I think we hit the top of the curve about two years ago for parts availability sadly, and now we're at the bottom of the barrel.
You can either keep struggling and repair the boards you have, and I know it's very hard, hence my flippant "and a new Lisa" - and indeed, I feel your pain, or save up for a new mobo, or wait until one becomes available.
The odds of a working 2/5 Mobo showing up on eBay is pretty low, most that will show up will be corroded.
There's another factor here, in that most of the 2/5 machines have had a battery leak at some point in the past, and while at some point they worked despite the leak, or were patched up, the alkaline contents got into the traces of the I/O boards and slowly over time ate those infected traces. So boards that once worked over time will, or have already failed.
Most of these types of Lisas that have been sold on ebay were chopped up into parts, and the parts sold, but if you see photos of them, most are very corroded.
2/10's don't have this issue at all, but 2/10 motherboards are incompatible with 2/5 chassis due to the wiring harness and require the 2/10 I/O board.
The only exception is that it's possible to use a 2/10 I/O board in a 2/5 if you use a modified Lite adapter.
If you try the reverse, a 2/5 motherboard in a 2/10 chassis, the internal widget cables would be shorted out when a Widget is connected, but you will be able to use the external parallel port - one of my two original Lisas was this kind of FrankenLisa which caused lots of other issues.
(I'm using 2/5 and 2/10 here to simplify - 2/5 ofc refers to a Lisa 2 that has a 5MB ProFile and 2/10 refers to a 2/10 Lisa that has an internal Widget, but I'm ignoring the storage medium here to be a bit more clear instead of just saying Lisa 2. AFAIK all the Lisa 2's were "upgraded" from Lisa 1s.)
2/10s are really nice if you replace the Widget with an Aphid/XProFile/IDEFile, etc. as Widgets are pretty flaky. There are also timing issues due to clock changes on the 2/10 board, and this causes incompatibilities with 3rd party devices such as some revisions of the FloppyEmu (and the in progress Aphid). The other advantage is that Xenix assumes that if you have a 2/10, the drive must be 10MB in size, which is a problem if you happened to snake a cable out the back to an external ProFile, etc.
But yeah, as in the other thread about the total number of Lisas, most of the existing working ones are at this point owned by collectors, the damaged ones have been either trashed long in the past (sadly) and there's very little stock now. So modern replacements are encouraged when affordable.
I wonder... I don't know if it's easier to repair an existing board than to design one from scratch based on the existing board and have some place print new boards and find replacement parts and then desolder the old board and solder the components to a new board (or use new components). Both are going to be a huge uphill struggle. So given that, I think what Vintage Micros offers is actually a great price. Sure, 10-20 years ago when we thought there were 100K Lisas out, and most were working, there, those prices wouldn't make sense, now they do.
Would it be less work to build/print a replacement PCB vs buying from Vintage Micros? How much is your time and energy worth? This guy seems to have done exactly this and even added SCSI to a Lisa 2 with a D-ROM (replacing the Serial B connector for some reason):
https://www.youtube.com/watch?v=Z5DraRVMVFM - but won't released his gerbers, etc.
For some of us, this situation is a lot like owning a house for a very long time in a city whose real estate has all of a sudden hyperinflated - like San Francisco and now the owners of said house are priced out by taxes and are forced to move - that is kind of analogous to not being able to find replacement parts to keep it working.
