Not very many. Bikes that use 559mm wheels are mostly MTBs and cruisers, with some touring bikes thrown in the mix.

If I was trying to do this on the cheap, I might choose a non-PT Pasela 26 x 1 1/4. They're a very basic tire and kind of heavy thanks to a wire bead, but thanks to thin sidewalls and no puncture protection belt, surprisingly supple and fast.
The Continental Grand Prix is available in 28mm 26er.
The fanciest skinny 26er slick that I'm aware of is probably the Rene Herse Elk Pass.

A lot information is missing here; what does it mean "bikes that I've been meaning to convert to 650c" ? Anyway, if you're looking for a 26" road tire, a good option is the Ritchey Tom Slick 26"x1.0" size (almost the same section of a 25mm rr tire)

Let's see some pics and detailed plans on the "conversion"!

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Some are born to move the world to live their fantasies...

https://triomultisport.com/
http://www.mjolnircycles.com/

Some cool suggestions, thanks everyone! I'd never heard of Rene Herse, very interesting company.
Brider, I don't have anything photo worthy yet but will post when the time comes. The two bikes are both beam bikes, a Trek Y-foil and a Softride Rocketwing. I haven't decided which one I'm going to convert, or maybe both. But now I'm thinking 650C or 559 may not be enough, and I should go 24", where there are also some good road race tires. Will be an interesting winter. Also planning to convert a 48' mower deck (just the deck, no tractor) into a battery electric hand-push mower using Makita batteries.