As far as I can tell, SeaCrop, Bill's Reef, Inland Aquatics, and Florida Pets all sell seagrass but whether or not they have it in stock or are shipping now is a different question. Also listed on there is marine fauna cebu, they are a distributor, on their site it doesn't say anything about needing to have an account with them to order but it is likely that you do and even if you don't you still need to make a store-sized order so it probably isn't worth your while (convincing your LFS to order from them is however an option)