Where to open a solo 401k?

[mo42nyy]

I know most people on here recommend vanguard for investments and I have them for my sep and roth iras. However this year I want to open a solo 401 k because its basically the same as a sep ira with the exception that you can invest a higher percentage of you earned income. (the max is the same, but you can contribute 100 percent of the first 15,500 dollars you make)
What other places with low fees offer solo 401ks?
Do they have similar funds ( I guess I could buy Vanguard funds through someone else but would prefer not to pay fees unless there funds are a lot better)

Also if I have anything wrong about solo 401ks please correct me.