I can find lamps everywhere, just no fixtures......

I haven't seen 40w circline bare tube fixtures for many years (at least 20). Home Depot has the round and square covered 40-32 ones.

You're right, they don't seem very popular now. We had lots of them in Australia about 25 years ago - but they are becoming rare. It seems the flush 'button' design is all that's available now (well, here anyway). I think the circline was a great space saving solution in most cases where linear tubes would not fit or were too dominating.