Look around and do what works.
In some places you can only buy from the CG you are in. We used to bring most of our own wood before the restrictions. Now it can't be brought into WI state parks from any more than 10 miles. And then if they see you with it they want a receipt to prove where you got it. I think they are getting close to restricting firewood in WI state parks to what is sold in the parks. Most of them are $6 a bundle now.
Lots of people in WI sell it by the roadside for about half what the parks, both public and private, sell it for. We paid only $2.50 a bundle for very large bundles near Egg Harbor this past weekend.