If you get Age Verified, there will be a fair few more retailer selection that open.
Not to mention the classifieds, full of other goodies. Most are packages, with mags, batteries, etc...
As for changing the spring. It's straight forward-ish, requiring some technical ability.
Check out www.mechbox.com
for some guides.
Right now they are doing some upgrading to their services. For the time being Youtube works okay.
Just look up V2 Mechbox disassemble, as those guns all contain a V2 Mechbox.