As Holmslaw said scrap metal dealers but also have you tried ebay or the immersion heater manufacturer - they may have one for sale/sell you one.
Other alternative is to replace the immersion heater unit itself.
5 minutes with my flexi rods and I can fish it out.
Get a longish strip and make the end all sticky (gaffer tape, the wrong way round, etc)
You'll get it.
Find something to cover it in the meantime. There's live exposed connections there.
One last tip. HAve a big bath to drain all your hot water and don't turn the hot water heater back on. Its easier to work over a cold tank than one that is at 70°
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