good choice on the gun... i was stuck between the 18C the M9 and the TM hi-cappa when i got my first GBB.........
what upgrades are you looking for, like more range, more accuracy or just some bada** externals?
heres a good site for the internals.
http://www.airsoftextreme.com/store/index.php?main_page=index&cPath=443_461^they are based out of cali, so the shipping is not gonna be that bad and you wont have to worry about the international customs stuff (usually not that big of a deal unless your shipping large quantities)
http://redwolfairsoft.com/redwolf/airsoft/Category?menu1=8^by far the best site for selection. they have alot more on GBB but i didnt want to paste 10 different links so just go to the 'tool bar' on the top right to navigate.
good luck. are you planning to get a rifle any time soon or are you just plinkin in the back yard?
sticks