It took a bit of digging on eBay, but I eventually found some Fixapart branded antennas, sold by a UK based seller and available in a decent variety of sizes. They quote both retracted and extended lengths as well as diameter, whereas a lot of sellers just quote the extended length and leave the rest to guesswork!
Possibly a bit OCD but I wanted the antennas to look as close to my boxes' original ones as possible; these ones have cylindrical chromed tips, as opposed to plastic or mushroom shaped.
They haven't arrived yet so I can't speak for the quality.
Cool JVC by way!