Metric sizes

I have a General Pump, ts2021, Tseries 47 that has sprung a leak at a plug in the manifold. The plug has a hex head and goes through a washer before entering the manifold. I got to the General Pump website where I got a parts explosion and a parts list. The list describes the washer as ' washer, M21.5'. The local Ace hardware has no such animal though they have a large assortment of metric washers. Can someone fill me in as to where I can locate such an item? The washer is non-ferrous. It has separated and the plug leaks reducing pressure quite a bit.

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