we dont have a dip tank, but ive had to replace a lot of the o rings in our MZX frames.
i bought the o rings from grainger: Round #111 Medium Hard Buna N O-Ring, 0.424" I.D., 0.63"O.D., 50PK
Item # 41UK09
i actually bought 50 packages of the previous item that was available at the closeout price - 41 cents per package.
i liked the plastic inserts a lot better, they seemed to stand up to years of use. the o rings arent a good solution.