Unless you have a rare Uniglide
cassette, it is quite likely that you are looking for a 6sp freewheel. In the first case you can just replace a hyperglide body and fit a standard 7sp cassette. It is also remotely possible that you have a Uniglide-hyperglide compatible hub (I have two of the kind, both similar vintage to what you indicate, an RX100 and shimano 600) - in such case just get a 7sp cassette, preferably with smallest sprocket bigger than 11t.
In the latter case, head for any bike shop (the worse, the more likely you find it