Well, if the one at Leonard and Sheldon's place is the same ole Shuttle that has been around since season 1, that one I believe is just a built Revell 1/72 kit (gear up with the flight deck section missing). I admittedly don't watch the show faithfully (only an episode here or there, although I did watch Howard "Fruit Loops" Wolowitz take his Soyuz ride), so I haven't seen what is in Raj's office.
Display models of about 1/200 or 1/144 scale (1/100 scale shuttle orbiter is the largest I've seen for sale, nothing bigger than that UNLESS it might be possible to place a special order) usually can be found at the NASA giftshops. They are not cheap though. Some of the better aerospace museums also carry them. Anything above those sizes IF they are display models from NASA was likely donated since one typically does not see them for sale. But sometimes if you scour the auction sites or can check around towns that have NASA centers (or the places where companies like Lockheed and Boeing build space hardware), sometimes you can get lucky if there is an estate sale or somebody is cleaning out their garage.
The NASA centers like JSC, Marshall and Kennedy have their own dedicated model shops to produce replicas for display purposes, in offices and stuff (wood shuttles with opening payload bay doors for instance). In one case, the Marshall SFC model shop sent Niel deGrasse Tyson a BIG Saturn V model as a thank you gift. But they do NOT do that sort of thing for the general public. Contractor companies also have model shops to make stuff like this as well, but the same things apply (i.e. not for sale, internal use only).
One excellent resource on the web to check out is collectSPACE.com as there are a few space collectors over there that watch "Big Bang" from time to time and they might have an idea where to find some of the rocket models.
Of course, if one has modeling skills, big display models of shuttles can be done with available commercial kits (such as the Monogram 1/72 shuttle stack). Although they may lack the pedigree of a real "Big Bang" prop
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