Nikwax makes it, and places such as REI and specialty outdoor-sports stores sell it.
I've been using it for many years on both Gore-tex and non-Gore-tex outerwear whose DWR finish wears off, as it eventually does.
I've even used it for the entire drysuit. Works fine, but after the suit is dry you should wash the gaskets with bar soap to remove the DWR residue.
Pull-Up Strap Handle Kit
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