Measure the stubs the spring sits between.
Go to a good hardware store with a spring assortment bin. You will find a drawer with compression springs.
Pick one with an ID that will fit over the stubs. Try and get one that looks about the same as the one you lost, but if in doubt, get a couple different tension weights. They will likely let you bring back the ones that don't work.
Once you get it all up and running again you will discover the missing spring sitting right behind the grinder.
Note - too much tension is not as big a problem as too little.
Tips on finding things in a cluttered shop:
1) Clean the shop out - Take everything movable outside and set it down on a big tarp. Blow off/out all the equipment. Clean out the shop of dust, scrap, and debris. Put everything back that you need and either store away or discard stuff that you don't. Toss all those odd scarps of metal and wood that you were sure you would need someday. That day never came and likely never will.
2) When looking for a thrown item, first look in the direction you thought it went. If that doesn't find it, look in the opposite direction. Sometimes it didn't really go anywhere at all and is right below/behind/next to where it fell off.
3) Look under cabinets and such. Things love to roll under them. In the same way, look behind things where the object may have fallen out of sight.,
4) This is the best tip I can give people - When something gets sent flying by a grinder, flex-shaft, or tool, close your eyes and listen. You will hear sounds as it hits things. Was it a metallic, dull, sharp, padded, soft, etc. sound? Look at the things that make that sound and follow the trajectory of the bounce. If you heard a sharp whap and then a metallic tap, it probably hit the wall and landed on a metal surface below that wall. If you heard a sharp cling and a very dull tap. it likely hit a machine or other metal object and bounced to the floor. With practice you will learn to discern the trajectory and even hear it skitter across the floor and under a cabinet.