Not sure where 'here' is, but there are on-line sources for Kerdi...check out the store on
www.johnbridge.com. I've used them for two personal projects, and it worked out well. They'll cut and send you just what you need, quickly. For the tub deck, I think you could also use Ditra, then you'd need either the KerdiBand, or strips of Kerdi to seal the seams to make it waterproof. Kerdi is designed to install over cbu or drywall...Ditra is designed to install over plywood or concrete. The point being that Kerdi should not be installed directly to plywood, and I think you'd want cbu there, whereas the Ditra could go directly over a plywood deck. Depends somewhat on how you layed the deck out.
If you have a Home Depot near you, or most tile stores, you could use RedGard from
www.custombuildingproducts.com. This is a paint-on (brush or roller) waterproofing material that would work in this situation. You need to tape and thinset the joints first, then after that cures, paint on the two coats of the Redgard. Once that dries, you can then tile the area. This would be easier to waterproof the edges of the tub cutout in the deck. It comes in a gallon can (or larger), so you should have plenty for this one project. The HD usually has it, but sometimes it's not where you'd expect it...