Your local DIY store. Screwfix, Toolstation, EBay, Amazon. Even friendly neighbours with a garage full of DIY bits.
You'll probably need a nut with a locking collar "nylock"
It'll probably be an M6, maybe an M5 or possibly M4 - figures are the size in mm.
If you can remove the bolt, try taking it to a local independent DIY store. Local stores will often have ranges of nuts and bolts in open boxes. I have taken bolts along and checked multiple nuts until I have found the correct one. Then you may pay just pennies for the nut you need. If the bolt can't be taken to the shop, take a similar nut off and take that to try to match it that way.
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Where to purchase a nut for garden chair bolt?
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