I don't know of any generic cases for TLRs, but you might try Photo Backpacker (see the link below). Since they sell case components, one of those might fit nicely. They also seem to indicate that some custom sizes for things might be arranged...but I'm just guessing about that. (They're also apug sponsors.)
I usually carry mine in a backpack or messenger bag inside a stocking cap. As long as it has a lenscap I feel pretty good about it but keep thinking I'll get a case from the photo backpacker place. The stocking hat thing just doesn't have the level of taste I'd like to maintain
A padded holster for a 35 mm SLR and lens will accept a TLR (even with lens hood in place if you insert the camera upside down). There might even be room for a couple of rolls of film and some filters.
I used to use a U.S Army small medical pack,the sort they used to carry field dressings in although I had the Yashica erc. because it was cheap and it didn't look like a camera bag .
I got lucky at the photo swap this morning and found an original case in quite good condition. Only $5. I understand the disadvantage when changing film and was mainly thinking of the case for transport, not so much for shooting while in the field.