As stated above mcmaster carr or also look up Hercules sealing products in Florida. I work for a crane repair shop and we order tons of seals from them for hydraulic cylinders. They will even do custom seals
I have had 2 cylinders go bad over the years. While I have never attempted to rebuild the items myself , I found Mec to be more cost effective (including shipping both ways) in the rebuild than local cylinder shops. In fact-- one time the internal spring broke (which caused tube damage) and Mec replaced the tube at a very reasonable rate.
Your choice & we all like to save $ , but I feel Mec was fair in price and fast in service.