Like an incoming missile ridden by a deranged Valkyrie in day-glo sports leisure wear, the thing they call the "onesie" has been a blip on my fashion radar for a while now.

That said, I assumed it would crash in the Atlantic before reaching my airspace. Wrong. As is so often the case, my sartorial judgment has been found wanting: I've spotted a onesie in the wild and at Aintree no less, where one of the less-well reported tragedies of the Grand National was the decision by an as-yet-unnamed chap to turn up wearing this strange garment, basically an all-in-one jogging top and bottoms. I can only assume he lost a bet or was on his way to a stag night in a secure government facility. Or, more likely, had escaped from one.

Even more baffling, his onesie was emblazoned with a four-legged creature which, while perfectly at home in Lapland, could only get round Aintree if it was harnessed to Santa's sleigh. The young man had the good sense to keep his hood up but that didn't stop photographers snapping him in his reindeer-print romper suit. The pictures were all over the papers the next day, at least the ones I read.

Now regular readers of this column will know that I'm a fan of the elasticated waistband and generally have a lot of time for sports leisure wear. But still, there are some things a grown man just shouldn't do, and trail round the shops trying to find a onesie is pretty near the top of the list.

If you haven't guessed what's coming next, let me tell you I headed to John Lewis first. "My good fellow," I said when I'd finally found the menswear section, "please bring me that very reasonably priced £139 Lusekofte Nordic Print Onesie in a medium. Navy if possible." Actually, I never got that far because contrary to what the website says, John Lewis doesn't stock onesies. "It might be a web exclusive," said the man in the tone of voice people use when they're trying to get rid of me.

Next stop, H&M. "What's a onesie?" said the shop assistant. I explained and she told me to try Primark.

I tried Topman instead. "We did have them but they all sold out," the girl said. "We did them in plain colours or with skeletons on."

"Sold out?" I said. "You mean people actually wear them?" She rolled her eyes as if to say "search me".

For the record, I did try Primark and once again found nothing. Same story in New Look and Urban Outfitters. I gave Harvey Nicks a miss.

My only hope, then, is web retailer the All In One Company. They specialise in design-your-own onesies and claim to have kitted out the cast of Big Brother recently. As nobody watches Big Brother any more, that's hard to verify. Still, if they can run me one up in maroon by May 19, I might even wear it to the cup final. Until then, the onesie remains something of a myth, at least as far as I'm concerned.

Perhaps an Aintree unicorn would have been more appropriate than a reindeer. n