Sure, but it's not cheap.
I found this page:
http://www.howcogroup.com/guide/002/o_431.htm
That says 132ksi, 11% elongation. Any further heat treat is going to increase the tensile strength, but the elongation will decrease.
I think that would be a good replacement for a grade 5. For Grade 8, it would depend on the application.
I certainly would not put those on in an "itelkillya" role unless I had a very good idea of the loads involved.