Oil dri is the best deal around. just look in the walmart auto section. paid 3.99 a bag for it in Hawaii. you cant even buy aquarium sand for that here. it is a little on the light side though, and gets blown around by the filter outflow.
Cashman, how has it affected your PH? its supposed to be inert like sms, but for some reason, the ph in my test 10g dropped from 7.5 to 5.3 in a couple of weeks with normal water top offs. there is nothing but the oil dri, rocks (inert, they dont affect water chemistry in the other tanks i use them in), and mosses. i set this up for some blue pearl shrimp, and figured the ph would stay slightly alki, like they like, but nope.... nice tank by the way.