I cannot provide much insight on alternate ways to find Rivieras, as my main one was purchased from a fella who lives about a mile away from me. My second one was bought from just random auto sales site searching, which I would recommend if you have free time... there are some gems out there on relatively obscure sites.
With regard to being concerned with purchasing one with over 100k miles on it, I'll give you my personal experience. I purchased my '98 with about 115k+ miles on it. It has been almost entirely a good experience except for a few maintenance issues that I should have taken care of sooner than I did. The previous owner took good care of the car and I try to take very good care of mine. For years, I drove the car a little every day and over those years, I virtually never had any issue except very minor ones (e.g. valve stem leak, new spark plug wires). Others may have some suggestions for you, but this is from personal experience, and it is one reason why I love the thing so much.