In my experience...contact the usual suspects (RD Enterprises, JAE, Dave Bean, etc) and see if they are putting in a big order from overseas; large shipments are charged at what's called "dimensional weight" (ie a standard weight for a box/crate measuring Length x Width x Height...the more you can cram into it (group buy, etc) up to the 'standard', the cost is the same.) Customs? That's another story, but, I've gotten in on a parts buy and paid my proportion of the weight even though my bits were part of a larger shipment. You can fit a LOT of juicy bits around a frame in a crate...
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Probably for liability reasons that whomever is building replacements won't sell 'subassemblies'; but frames are out there, may need some repair, but, as others have said, the crossmember is usually the first to get hosed...the rears tend to be rustproofed by years of oil leaking, and the long box is the last to suffer.
I took two frames (ok, they were S2s...) where one had a roached front, the other a roached rear, and brought to a well-known fabricator shop with the measurements and angles, and he spliced together a decent front cross member to a decent rear and came up with a frame that was within 1/16 of the blueprint figures. But, he had a surface table to do it on.
It's not impossible, but it does take time to search out what may be available. Here and the Lotus Europa groups.io mailing list are good places to start your search if you go that route.