I love when big artists are "don't undersell yourself!!!" "Just advertise yourself more!!" "That 100% works!!" But never tells you how lmao
In my personal experience (it's not like I had hundreds of comms but I got a fair amount) +

+ instagram (very few), twitter, there's a site/app called wix where you can make a portfolio and that stays online, meaning that other users can see it, I had like 2 commissioners from there but it worked ¿ Reddit, I used it only once bcs I'm lazy but it also works +

I know there are subreddits like r/artcommissions or commissions or something like that sorry I can't remember how I looked it up and I forgot my account JAKSJS but If I log in again I'll add it here
Tbh reddit was hard for me to use I'm too lazy
Tumblr too
+

+FACEBOOK GROUPS personally I haven't tried it yet bcs I always forget (I'm sorry) but my friend told me she always get commissioned there, they're basically like artists groups or commissions groups, I've seen they're 24/7 looking for artists to commission so

If you're not lazy like me and can be active in fb I recommend you to look for those groups
And maybe deviantart? Those are all the sites I know¿ it's more about trying a lot of different sites instead to just stick to one

I can't remember anything else sorry I just woke up and chose violence
BUT HOPE THIS HELPED ANYONE
I mainly did this bcs I remember one of my friends told me she only advertised her commissions only on ig and I was like nOooooO
And I thought maybe more people do it like that¿