Does anyone know where I can obtain a V - spring for a 1978 Miroku 800 trap gun, and a rough estimate of cost?

Unfortunately the best source, BC Gunsmithing, in AZ is gone. Every once in a while someone will part out a Miroku, but it's usually because they can't get hammer springs.

If you can find a real gunsmith you might be able to get one made.

Dunno if they are still available here or not, but try emailing countryway gunshop in the UK. If they are available then these chaps will know, and where to get them from if they are.

I don't know what your spring looks like, but many over/under leaf springs are quite similar. There is pretty good chance something else will work in it.

A couple of years ago, I replaced a broken top lever spring for the owner of one of the preserves where I work my dogs. It was an early 20 ga O/U and had a pin looking projection as part of the V spring that fit into a hole drilled into the receiver. If it is that top lever spring that I got from Bobs, a generic spring will not work. Hammer springs are a different matter, and there may be something out there that will work. Brownell's has some shotgun V springs in their catalog.