I've still never played one, but I'm also fascinated by church basses. However, I seriously doubt it will sound anything like what you're used to. I don't care about those things like purists do, but since you're playing classical music... well.
Some time ago (as in, a year or two, I think) I was emailing with Kolstein, and he said they had a church bass in the shop that had been converted to be strung and played in a conventional way. The price was around 10k. I would have followed up on it, but getting over to the US hasn't happened since.
I remember that the Cincinnati bass cellar had one in unrestored condition around that same time - I can't remember at all what the price was.
All of that said, check these out:
Historical Index Musurgia - Fine, Rare & Peculiar Musical Instruments
They do also come up on ebay from time to time. Trolling around there the other day I did see this interesting "cello":
eBay - New & used electronics, cars, apparel, collectibles, sporting goods & more at low prices
Good luck!