Hi, I had the same problem and the ones I got from the club turned out to be track rod end boots, far too small. The club offered to refund but I kept them. I looked on ebay and the nearest ones I could find were 18.5 at the small end and 39mm at big end. I checked a brand new upper balljoint and that was fitted with an 18 x 40 which to be honest was a bit short and was fully stretched when the ball joint was moved fully to one side, though that probably will never happen in use. Anyway I fitted one of the 18.5 x 39mm boots and it fits well, though a bit slack in the pin the pin is tapered and when pressed fully down by the upright when tightened on seems to make a good enough seal. Chances are these are the same boots as supplied by the club.
There are some silicone 18x40mm available from a chap in Bulgaria for £8 a pair with free postage but I don`t know what they are like.
Ralph