You're going to have to open that floor in order to find the source. Sorry, I know that sucks. Until you find the source and the extent of the damage, you just don't know.
A couple things - I asked my Realtor DH about the asbestos. 1975 is unlikely to have asbestos, but if it does, it will greatly increase your costs. AND - if you get positive test results, you must disclose to potential buyers. You will almost certainly have to repair this before selling, unless sold for tear-down.
The idea of a home inspector is a good one - with reservations. Home inspectors _can_ do things like cut holes, ect if hired by the homeowner. They can't do what is called "destructive discovery"; (ie damage anything) when hired by a potential buyer. That might be a good idea for you, since they have no state in the repair. (They tend to be a good source for contractor/handyman recommendations.) But they would most likely have to take down vapor barrier and insulation to check, which you would then have to replace/repair.
Real estate law varies by location, but here you would have to disclose to potential buyers anything the inspector reports wrong with the house. So - be wary of getting a full home inspection and perhaps just have them do the problem area?
Also, I would wonder about the contractor not wanting to go in the crawlspace. This repair will require crawlspace work.
Could you contact your home insurance company? Depending on your policy, some of the repair might be covered.
I'm sorry you have to deal with this mess.