Is there a place I can find a list of known unsatisfiable cores for X variables [no more then 10] in CNF format?

Or is there an 'easy' way to find out, say I have 7 variables how many clauses [of the 7 variables] do I need for an unsatisfiable cores.

[I calculated 2-5 by hand, I am just trying to figure out 6+].

Thanks

Staque