I bought the Vortex D-1 from BigAls and it works great and the price is awesome.
order any extra D.E. from them ,you can buy a 10 lb. bag from a pool supply place for like 6-7 dollars... aquarium grade and pool grade are one and the same.
However, do yourself a favor and order the DIATOM RECHARGE VALVE(Vortex) as listed at BigAls for 13.99...
Its the best money you will ever spend on a filter part !
It is NOT necessary but the filter is a pain in the butt to use without it. Its used for priming the bag with D.E. which can be messy.
There is no picture of the valve at BigAls but you can see a parts breakdown at the Vortex website.
Did you see this yet ? Photos of a Vortex in action on my tank, this shows what they can do !