I had exactly the same problem. I had to go for a 400mm zoom lens in the end to get anywhere near what I wanted. The teleconvertor cost something in the region of £350, the Zoom lens in the region of £800.

I would have thought that any decent retailer would accommodate your request Bob, as it is in their interest to do so. Just make sure the lens is returned to them in it's original packaging, so they have no excuse to renege on the return.

The biggest problem with the 400mm though, is the weight. You really do have to cart the thing around when you are out for the day.