Well, for the application I was looking into this for (remote deployments in Greenland and Antarctica) it would be almost certainly politically impossible, so d

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).