It is unfortunately much harder to do DIY via testosterone, as it is a controlled substance (in the US, it is a Schedule III controlled substance). Estradiol and progesterone, on the other hand, are not, so they’re much easier to source. The only places I’ve heard you can source testosterone injections are…shady at best. Think hidden onion services through Tor, the same place you can buy even less legal things; the dark web. You aren’t going to find legitimate online spaces because distribution is a major criminal activity. It’s unfortunate that trans men are much more limited in DIY, but if you’re looking to source testosterone, your best bet is to ask the right people in person. You can find resources on how to dose and inject online, it’s just much, much harder to source.