Well Julie, it certainly isn't the altitude itself that's the issue, nor a lack of UV, but I'd first get a water analysis from your provider, or if on a well, get that tested.
There are lots of plants that like the cooler temperatures, but few really do well in low-humidity conditions. Pebble trays won't to anything to help that (
read this), so you really ought to consider a better way to boost your RH - 50% is better for them, your health, and for the structure of your home.
There is a vendor whose marketing phrase is "orchids on a stick" that has a pretty good search tool, allowing one to plug in cultural parameters and search for likely plants, so will give you some good leads, even if you don't buy from them.