"A Well moderated board that is a benevolent dictatorship, not a democracy" In business since June 16, 2010
Got an Ace Hdwre near you? Gaggles of springs, usually displayed on a board, and located in bins.
Here's a couple amazon options, both metric, but within your criteria.
I just dug through my box of random spring and only had 3 that fit reasonably over a 1/2 rod, all of which were grossly outside your other dimensions.Mcmaster has the best search function for things like this. You can sometimes narrow an item down in their search, then find it elsewhere (amazon or ebay) if you don't want to pay their 2nd day shipping charge.
McMaster Carr.