If you want friction most any stem mount shifter would work but I would go for the Suntour Power ratchet ones.
The very basic, unfortunately mostly plastic, shipmano SIS should be around in the 6spd variety.
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“One morning you wake up, the girl is gone, the bikes are gone, all that's left behind is a pair of old tires and a tube of tubular glue, all squeezed out"
Sugar "Kane" Kowalczyk