What stage of bonsai development are you interested in purchasing?
1 -Seedlings?
2-stock in nursery containers, 1 gallon or 3 gallon or ? larger sizes?
3-recently collected trunks? What diameter? what height?
4-Pre-bonsai ready to move into first bonsai pot? Shohin? Larger sizes?
5-Bonsai hornbeam that is already styled and in a bonsai pot, has been developed enough that display at a show is possible in as little as 2 years to maybe 5 years?
6-Bonsai hornbeam that could be show ready in less than 2 years?
7-Bonsai hornbeam that is show ready?
Different people would be good sources for each option.
Once you get to the more developed stages, the price for the trees will increase dramatically.