I found another material that I can buy freely and test with my Radiacode 103 radiation detector: WT20 Welding Electrodes.
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These are tungsten welding electrodes with 2% thorium. Thorium is a little bit radioactive and shows up in the spectrum that I measured over a period of 3 hours with a Radiacode 103 device:

Article Link: Quickpost: WT20 Welding Electrodes & Radiation | Didier Stevens