I acquired all my tubular O-gauge track second-hand through train shows and private sales, but it took a while to find the full variety of O31-O42-O54-O72 curves. Some of the track needed a lot of elbow grease to clean up. Buy new track if you prefer to spend money instead of time. I bought a good variety of clean used O27 wider-radius track off this forum.
One other uncommon size: prewar American Flyer O-gauge had O40 curves. It's compatible with Lionel tubular track.