You could either rent or buy an iPad or iPhone, check Craigslist or look for refurbs if you want a good deal. If your website generates a lot of income from iDevice users then you should probably bite the bullet and get one for testing. When I needed to test once I borrowed a friend's unused iPod touch which has the same browser as an iPhone.
The amount of shopping from mobile devices is skyrocketing so it's not a matter of do you need one to be able to services your customers, you need one to make sure they're getting the experience you want them to have on your site.
Ultimately I ended up buying a smart phone and a tablet, but I went all Android so I can't help with any iDevice issues, sorry. ;)