20 years ago, UChicago Scav Hunt had an item on the list: "Item 240. A breeder reactor built in a shed, and the boy scout badge to prove credit was given where boy scout credit was due. [500 points]"
Well, for the application I was looking into this for (remote deployments in Greenland and Antarctica) it would be almost certainly politically impossible, so did not pursue, but I doubt it's possible to purchase Sr-90 in non-negligible quantities as a civilian (you can purchase it as a calibration source easily enough, I think, but at great expense).
I found some speculation that you might be able to build one out of thorium from smoke detectors and lanterns, as in the David Hahn case, but not much. No way this would be legal though.
I've never heard of thorium being used in smoke detectors. As far as I know it is always Americium, including in David Hahn's case.