I usually just make my own banners, but sometimes I'll commission banners from other people and in rare cases people reach out to me and make a banner for my forums. I'm not the best when it comes to graphic design though, I use Gimp whenever I make banners.
99% of what I use, I typically make. In the past, I've been involved with graphic communities and relied on them to create me something if I couldn't design something that I felt was worth using, but nowadays I typically make everything that I use.
My good online friend who is a web designer creates all the graphics necessary for my forum, she's too kind. We've been working on skins and converting them for probably over a decade at this point. She was always the graphics designer and I was the coder.