Yah, Shimano makes nice pedals, but those environmental seals tend to be pretty cheesy. It's important to keep those clean and put a drop of light lube on them on a regular basis. The main reason they deteriorate is that they dry out and stick. Once they start to stick or bind, they often break or disintegrate. A little ExtraDry or TriFlow is the solution.
Using an o-ring is a good suggestion, if you can't get the seal. Just make sure and grease it when you put it on. Then, as above, keep it as clean as you can and keep it lubed.
All that being said, you should be able to get those seals from Shimano, by way of your LBS.
Last edited by cascade168; 09-14-08 at 12:39 AM.