I bought some off the shelf electrical parts from Autosparks last year and they were struggling with Covid, but did great service considering.
I purchased some of their correct Lucas pattern grommets, which really finish the bulkhead with the correct period look, you don't want to have to use metric equivalents they look very different. ..and of course you'll need them at the point you install the loom, it's difficult to add them later.
However on the subject of renewing grommets on an existing loom without unthreading the whole loom; I was faced with this task last year. I cut the grommets on one side with a brand new razor blade having cleaned the protective oil off it. I had already tested 3 different glues on some metric grommets and asked my son to pull them apart in a blind test. To our surprise we found that super glue works the best, very well in fact and remains flexible. I think this is because there is such a thin film if the cut surfaces are flat and newly cut. So I glued the grommet back together over the loom, allowed it to dry whilst holding it together, then pushed it into the hole, rotating the 'join' out of sight.