First, when a group starts a shoot that is period clothing and flintlock only to shoot. Do you really think that their main goal is to have more shooters to show up with non compliant firearms. They are not "unwelcoming", they have set what is important to them (usually historical context) and will welcome all like minded individuals with the desire to participate.
The people I personally have been involved with have given articles of clothing and loaned/sold flintlocks to newbies who have interest in the shoots and time period specified and as long as the newbie is showing an effort to fit in almost all flaws in their kit are overlooked.
Maybe I am lucky, I have never seen this "unwelcoming" attitude when people show up, ask questions and show interest.
Second, if you go and buy all hand sewn, period correct fabric, custom built flintlock, yes it will cost you big bucks. BUT almost none of us ever started there, we borrowed shirts, wore cheap crappy shoes that gave us blisters and gradually over time improved our kit until it met OUR level of satisfaction usually with quite a bit of encouragement and guidance from those who went before us.
You guys that denigrate the reenactors because all you want to do is be able to shoot in the BP only deer season........