Amazon has a lot of them. Just search for "magnetic usb c". The Planck can probably use the ones intended for the Macbook, but I don't know for sure (might only work for the low-pro case). Here is the one I have, which is narrower for broader compatibility.

These work rather well. SMART&COOL [ 2 x 6.6 ft L-Shaped, 2 x 3.3 ft ] GenX 3 in 1 Magnetic Charging Cable, Support Max 3.0A Charging Current & Data Sync, Blue https://www.amazon.com/dp/B08977X8HT/ref=cm_sw_r_cp_apan_glt_i_B682JZ4YJWJ4P3DE336J

Bonus tip: The Planck's USB-C port sits flush with the side of the case, so you'll probably be able to use magnetic connectors that are intended for the Macbook. The BM40's port is a bit inset, which means you'll either need a straight-on magnetic connector or you'll need one that sticks out a bit, like this one from Amazon (the one I'm using).