If you are buying on the internet anyway because your local doesn't stock them, why not order both and send the one that doesn't fit back? Obviously you can't wear in the water but wearing them out of the box shouldn't be a problem.
Sure you'll pay a few bucks in return postage but probably less than the petrol driving all over to find both sizes in stock and at the end of the day the shop still gets a sale. Probably a bit cheeky to order both to try on if you don't think you want either but if you can't decide which but want to buy one then ..... no harm, no foul?
That said, have you tried asking if your local shop can get them in? I know that most of my local shops have a relationship with suppliers that they might not routinely stock and will order in if I ask them.