As you have seen from my previous post, new guns are not cheap. In the 18th century you could probably pick up an old matchlock or wheelock for very little and a flintlock trade gun for maybe 4-5 English pounds. These days a cheaper alternative would be a gun that comes in kit form that you put together yourself. Another alternative would be a second hand gun, especially a mass produced product in percussion. A close friend of mine bought a percussion second hand not too long ago, and as luck would have it it was a CVA, and I had a new flint lock to fit his rifle. The locks are interchangable. A lot of people purchase muzzle-loading guns thinking it will be a lot of fun, and it is. But they don't take the time to learn how to use them properly and how to look after them so they end up getting rid of them at a loss. Always worth checking out your local gun shop.
A CVA rifle in kit form.
Another alternative is a bow. I have no documentation of non Indians using a bow, but in our group bows are allowed.
If you can't afford to purchase a gun or bow, then you can try making a survival bow, or you can simply carry on gathering the other tools that you want and wait for a bargain to come along. You do not need a gun or bow to get involved in 18th century living history or historical trekking, though in some parts of the world carrying a gun for protection is advisable.