Where you find parts depends on what parts you need. What are you looking for?
Most parts you will need are not specific to the P30 and can be found at chandleries. Engine parts are dependent on the engine you have. If you have an Atomic 4 check out Moyer Marine. Great resource.
Dan Pfeiffer used to have pearsoninfo.net as a great resource. It no longer works, but he does still have P e a r s o n    1 0 M
which is great for Pearson stuff in general.