Reaching out to your local university or Indian* reservation would likely work. Both are going to have good information about the past inhabitants of the region, and an interest in sharing that knowledge! I'm pretty sure American laws are different than here in Canada where archaeological finds like the arrowheads you found belong to the province or territory to be repatriated or stored, but you should also consider either leaving them in the ground or offering to return them to the local band. Archaeological best practices these days include leaving artifacts in situ if they are at no risk of destruction where they are.