when you go you will know where the term flash in the pan comes from
Do note that the CSA guys tend to be very hardcore. And they tend to outnumber the USA.
If you just wanna play, you can get a decent uniform from eBay, etc.
They Union are not gonna give you a hard time about not being 100% authentic. But yeah, tennis shoes are out. 
Some women strap down and fight as guys. The CSA dudes dont like that.
Easy to show up, talk to a local unit and have one of them show you what to get. There are usually sutlers there.
I spent a fascinating evening talking to a Civil War re-enactor once. There are degrees of participation. Some go all out, authentic down to their underwear, sleeping on the ground, and eating grub around the campfire. Some are bedding down at the local motel and getting a good night’s sleep. If you want to be truly welcome, bring a cannon. Apparently there’s always a shortage of cannons.
Thank you all. Very helpful.