The //c and //c+ can use any composite-connector Apple monitor out there.
The most commonly used "//c" displays are the:
Apple Monitor IIc:
Apple ColorMonitor //c:
I have one of each. The Monitor IIc "looks" more appropriate with the //c (the stand is nice, it lets it "float" over the //c.) But the ColorMonitor //c seems more appropriate to the //c+ to me.