Zeller is a material very commonly used in the cooking profession. It is used for the Zeller Aether Jelly recipe. This recipe makes food that restores 2000 DP when you use it. This food is very commonly used by players and sells quite well on the AH. Zeller can be farmed in quite a few different areas but one of the most un-common areas to see other players is in Beluslan just north of Red Mane Cavern.
Up here you will find level 39 mobs, quite spread out allowing you to easily get any Zeller you'd want without agroing them. Also while you're waiting for the Zeller to spawn up here you can also harvest Ruby and Mithril as well as Orichalcum.
Central Brusthonin is another great spot for you to farm for Zellar. You can find many "pockets" of it all over the place. My favorite places are to the far northwest of the picture, where I am standing currently as well as the Baltasar Cemetary area. You'll get the most bang for your buck in these two places. Also while farming for Zeller out here you can also get Mithril and Orichalcum.