Tirerack specs that as trailer tires. They do offer an 18" version but my tires are fine. Whats going on?

I'm a little puzzled by your post - a P205/75R15 tire will fit any 15" rim with an appropriate width, perhaps 4½~6 inch, as for Tirerack saying that it is a trailer tire, how about a link to that - the particular tire you were looking at may have been a trailer tire, but the size is not specific to trailers, and in fact the P prefix suggests a passenger car tire - 205/75R15 may be a common size for use on trailers, ST205/75R15 is a 205/75R15 tire intended for use on a trailer.  Tirerack does have several different P205/75R15, including one of my favourites, the General Grabber A/TX

Aftermarket rims are out there, 15x5.5, 15x6, with the correct bolt pattern and offset, tirerack may not have, but 4WheelParts does.

When I type in  wheel size the site defaults to this:  no mention of rims.  https://www.tirerack.com/tires/TireSearchResults.jsp?zip-code=91766&width=205/&ratio=75&diameter=15&rearWidth=205/&rearRatio=25&rearDiameter=15&performance=ALL

When I type in model, i.e. 95 sidekick it only give me options for 18" rims.

Tirerack.com is only one of several online sources, if they don't have what you're looking for, try another.