BNM custom sells upgrades for the Discovery such as custom multi-shot breaches and shrouds etc.
Mountain Air is another one.
Don't know if you'll find anything local.
I agree with Smaug btw. I haven't been able to find an upgrade path for cheaper air guns that ends up being worth the time or cash. They end up being as expensive as buying a gun that comes with the features you want, or the upgrade options are too limited to make it worth starting.
If you have to upgrade vs buy what you want straight away, the Marauder is the best place to start because, with the parts currently available, you could eventually end up with something quite nice.
e.g., for a 22 Marauder, you can buy a premium barrel (Marmot Militia), a bottle adapter and metric reg (JDS Airman), a carbon fiber bottle (Talon Tunes), and aluminum modular chassis system (RAI), a hammer de-bounce (WAR). You have to like modding though because, when you look at what else you could get for the same cash....