It took quite a while to find the relatively safe radiation source. Thorium mantle from camping supply store is usually sold and used for camping gas lamps, but does emit some alpha radiation. Alpha is relatively safe, because it can hardly penetrate a sheet of paper or a plastic, so as long as I don’t eat or inhale it I should be fine.
Actually, the usual dosimeters cannot detect alpha, but can detect secondary gamma and beta emissions which happens when alpha strikes something. Therefore, I did not expect any precise measurement, but it was quite reassuring to get the numbers pretty close to the cheap Chinese dosimeter I’ve borrowed from a friend of mine.