What instead of Sram Spectro s7 shifter
Go to your local bike shop and ask for something used out of their parts bin. They’ll often have shifters left over when someone upgraded.

My bike shop would throw in the part for free if I paid for labor, but in either case the cost would be trivial. It’d be worth it to get the derailleur cable (both inners and others) replaced at the same time.

Reading Sheldon’s page on Sachs/SRAM IGH it appears that the cable pull for that particular seven speed internal gear hub may not be standard. It may be better to just replace like with like. Again, your local bike shop may have the spare part in their parts bin. A local maker space may be also able to fabricate the part out of nylon or abs using their 3d printer. 

If you just need it to work, a friction shifter will work with just about anything. Your bike shop should be able to give you one for free or close to free.