Hi, Richard.
We just got DSL at our location. We still have the ISDN for voice only, until we get everyone used to our new phone number. Anyway, we always had a separate ISDN ISP. We were paying $25 a month for 128K with heartoftn.net, but just about the time we signed up for DSL, I saw a provider that had it for $9.95, go figure.
The secret is to look carefully at the nationwide 56K dial up providers. A few of them also have ISDN access. They don't seem to advertise it, possibly because they outsource their tech support and don't want to pay the extra fees or specify ISDN support.
Anyway, there is at least one out there at $9.95 or so, possibly more. If you don't mind paying more and require good support try isdn.net in Knoxville or eskimonorth.net. You can also try heartoftn.net, but use someone else's DNS servers. Theirs seem to go down fairly often. Level three is one choice. OpenDNS is another.
One way to locate national providers is find the ISDN numbers in and around Knoxville, then do a search on the numbers along with "ISP." That will at least get you started. Eskimonorth used to support every ISDN number in town. I assume they still do, so I would start with them just to get the numbers, then go from there.
Good luck.
Edit:
One last thing. If you think you can't get DSL because you plug your number into fastaccess.com and it says it isn't available, be aware that it lies. It is an automated system that literally tests the line. Since you have an ISDN line, it will say it isn't available, even if it is, because DSL requires a POTS line, which you don't have. It is a broken system. I discovered this when it told me I couldn't get DSL, even though they had called my neighbors and asked if they wanted it. They didn't call us, even though we had signed up to be notified.
I called to sign up and got the same song and dance until I told them to pretend we had no phone service and to use the street address instead of our ISDN number. All of a sudden the CSR's screen magically said it was available. Try the same thing and see what happens.