The first link should give you a variety of formats, at least one of which is importable to Inventor. STEP would certainly work, but I think they also have Inventor's native format (we use Solidworks, so we download the native files :-) ).
One quirk with the site is that you need to use Internet Explorer or it doesn't properly select (it might even be generating) the cylinder model according to the specs you enter in.
It's also perhaps a bit tricky to navigate, but you can search for a part number (haven't actually used that, not sure how well it works). If you're looking for the line that they offer for free to FRC teams, that's under Non-repairable stainless steel cylinders/Original Line. They also have a complete catalog of their pneumatic actuators at
http://bimba.com/Catalog/Download/, which may help you find the model you want to use in your design, nail down the part number, and then nab it off their CAD site.
Hope that helps.