I would imagine that it is more economical to culture your own phyto than to purchase it. Once you have a culture going, all you need is light, air, and clean water. One tip, never ever allow a single rotifer into your phyto culture or it will reproduce to epidemic proportions and clean it out. Some authors go so far as to recommend that the phyto cultures be kept above the rotifer holding area to prevent any from jumping or dripping into it, and always using separate tools for dealing with them.
Someone stop me! Please! But... not just yet...
55 Gal Tropical FW
55 Gal Mixed Reef