We have an odd sized bed, between King and super King, which I bought in a sale ages ago, without thinking about fitted sheets. Eventually I found a company that makes any size to order, but they are expensive and when one of the last lot wore out I just went and bought a nice pure cotton flat sheet in superking instead. And then wondered why on earth I hadn't done it before. Perfectly comfortable, never comes untucked, and folds up beautifully, not that I'm overly bothered about such things.
I would still need to get a mattress protector made to order if the current one ever wears out, but touch wood it's still going strong.