Can I substitute Ener-G Egg Replacer with Orgran No Egg?
Based on their ingredients they should be pretty similar (Orgran: Potato starch, Tapioca flour, Calcium carbonate, Citric acid, vegetable gum (stabiliser): Methylcellulose vs. Ener-G: Potato Starch, tapioca starch flour, leavening (calcium lactate, calcium carbonate, citric acid), sodium carboxymethylcellulose, methylcellulose). 

Do you have any bulk food co-ops / health stores in your area? Sometimes they sell Ener-G or similar products in bulk, that way you can just buy the few tablespoons that you need and try it out before investing in the whole box.