7 speed freewheels are still very easy to find cheaply. Shimano still makes them in the range you need. If you cruise eBay, 7 speed shifters come up from time to time. Create a search then save it to get updates. I have found old shifters as part of a group or incomplete group, often from people who are just resellers and don't know what they have. They can be pricy given their age. Or you can just go 7s friction.
If you're mostly commuting, you can get a
new 6s 14-28 freewheel for cheap. I would spring for a 38t chainring to get a bit lower gearing and you're good to go - nice even spacing in the FW. Think about the last time you
really needed that 12t sprocket... Unless you're in beast mode I'm guessing you'll likely spend most of you're time in the 52 and the 16t or 18t sprockets on the flats.