First, obviously stop shooting it.
Second, the best seals and springs are to be had from Jim Maccarri.
http://www.airguns.citymax.com/page/page/251327.htm The site is currently off line for maintainence, but once its back he has a section for FWB Sporter parts, including seals and springs as well as entire tune kits. It may not be the seal, sometimes the spring can break too. Best bet is to order one of the tune kits so as to get everything new right off.
Not a real easy gun to work on, you'll need the proper spring compressor to open it. There are some tricky parts to navigate around, including the sliding safety/spring that needs to be carefully removed during disassembly.
If you have never worked on one of these and don't have the proper tools to do so safely you might want to consider sending it to someone who does.Outside of springs and seals, parts for these guns are not easy to find and fairly expensive if you do find them.
When you order the tune kit make sure you get some spring tar and moly lube as well. The tar goes on the spring ( only a little bit) and the moly on the moving/sliding parts.
