You can get replacement parts for the AeroPress from many sources, but it might be more effective just to buy a new unit and get everything fresh.I use an Aeropress every day, but the seal is starting to go and it makes a mess more often than not.
AeroPress Parts Archives - AEROPRESS COFFEE MAKER
This source is in Canada. They specialize in Aeropress parts and accessories but are not affiliated with Aerobie.
Their prices are the lowest I could find, and shipping to USA is very reasonable.
I like that they sell the Tote Bag separately. Having a tote bag would make storage much more convenient.
They also sell other products like mugs and grinders that are specifically size-matched to the Aeropress.