my facebook page, etsy, DA, etc.....basically what PKW said
but price and quality also play a factor
and I can't think of a single MLP customizer who does it as their JOB, even part time (it's a hobby). It pains me to even think about how much I'd have to charge for a custom in order to cover my TIME at anything remotely close to min wage /hour and I've been doing this since the 90s lol. So....what I tell people is to make customs because YOU want to make them and NOT because you want/need to sell them (that's a bonus), and if you do make to sell don't expect a profit (maybe over supplies, but not over time)... and you'll definitely need an established portfolio to get people to commission you.