I have bought these two sets from Amazon:

https://www.amazon.com/dp/B07NHMCR9F/ref=cm_sw_r_cp_apa_glt_fabc_SBPQR5P7SJ1DZF7V85BS?_encoding=UTF8&psc=1

https://www.amazon.com/dp/B074SYK3H4/ref=cm_sw_r_cp_apa_glt_fabc_NQ1PTC6GPQXJBCMTDVC7?_encoding=UTF8&psc=1

I've found that the smallest size are a good size for magnetisize arms on infantry. The larger ones are more appropriate for vehicles and the like.

First born magnets: MagNetSol Magnets + Drill Bits... https://www.amazon.com/dp/B074SYK3H4?ref=ppx_pop_mob_ap_share