Several sources come to mind, but you really need to be more specific about what you consider to be a mid-level kit, and more importantly, what you really want to build, and how much you want to spend.
If you read some of the older posts here, you will find that I, for one, don't think there is too much available in the "mid-level" catagory.
IMO It seem that there are Factory Kits which are basically factory production guns which are about 90-95% finished, and the the "custom" pre shaped but rough stocks with a box of parts like unfinished barrels, butt plate/trigger guard sand castings, etc.
The prices vary a lot. The Factory gun kits can be in the low $200s to the $600s.
The customs can be from $338 on up.
The $338 would get a plain maple half stock with a 36 inch Green Mountain barrel, a Precussion lock,Precussion drum, a simple trigger, sights, butt plate and trigger guard from Pecatonica River. In other words a plain, rock bottom priced rifle made with excellent parts.
If you have a Thompson Center "Hawken", you can re-stock it for $120-$250 from Pecatonica River, depending on the grade of wood you buy.
Let us know more about your project for better guidance.