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if you can still get them?
You can't get the Surly one new, but I know somebody got the Walker Brother (can't remember which brother) copy fairly recently NOS, so they might still have one or two even though they don't advertise them any more.
Bear in mind that they only work with a limited selection of mainly older or lower end Shimano hubs. Most 21st century ones with a 10mm axle should be OK (check the EV PDF for a visual on the spline type, if it looks like an FH-6600 you're good), anything with a bigger axle won't be.
There used to be a fixed gear converter that replaced the freehub and dropped into the splines in the hub. Anyone remember what they were called and if you can still get them?