I don't think anyone makes them.You could probably get a machinist to make what you want.I can tell you this,from a lot of experimenting,if you want front end weight,weight in front of the shaft has way more influence than weight extending back some distance behind the end of the shaft.
Test it yourself.If you want 100 grs more weight up front,put that much on a broadhead and find the balance point of the arrow.Now,instead of the weight on the broadhead,add the weight behind the insert.The further that weight extends behind the front of the shaft,the less affect it has.
If you thought you needed 100 grs extra weight on the broadhead,you may need as much as 200 grs behind the front of the arrow shaft to reach the same "tune".
I guess what I'm saying is,if you can achieve the weight you need,in the broadhead,it won't take as much.Either way,if you need a longer,heavier adapter,I think a machinist could make you what you want.
Tuffhead has some glue in "adapters" that take the place of an adapter and insert,in a variety of weights.You may have already seen them but if not,maybe you can find something there that would work?