Any Shimano EIGHT speed S.T.I. will work. You just screw don't the limit adjuster screw on the lower end of the gears so it can't overshoot.
Knowing it'll do 8 speed S.T.I. there's a whole lovely world of secondhand 105s and good quality Tiagras out there.
I'm not entirely sure but I think you can use a 8 spt sti with a 7 spd freewheel as shimano has the same spacing in 7 and 8, (minus the cog of course). You only have to limit the deraileur to the 7 speeds.