http://protective.sherwin-williams.com/detail.jsp?A=sku-26170%3Aproduct-6841
thats the stuff i used. Though since you won't be going back to that store it's a moot point. However, the product is amazing and well worth the 120 total price if you are wanting to make a plywood type aquarium. Other products will work, I've heard too many bad tales to recommend drylock. Some have had success using hardi backer along with drylock. ymmv.
Hopefully you can find something you want. I find it odd that they wouldn't offer to have it shipped to their store as they have done so for me. GL at any rate.