They exist, but they're pretty expensive - thee combined cost from Mr CAD is £499!
The cheapest options are extension tubes or close-up lenses, both of which start pretty cheap indeed, and go through to fairly dear. I got a set of second-hand Jessops tubes for £25, and a rather superior Canon 500T close-up lens for £35 as "new old stock", and that I've used for some years now for butterflies with telephoto lenses. Are the cheapos on ebay worth buying? Don't know, but I suspect they are only if you're not too picky about quality.