For those looking for exterior upgrades I found some sweet looking ones on Airsoft GI.
For M4 users
It can be as low as $30 per upgrade, with your choice of colors: Tan, Black, OD/Foliage Green.
Magpul is what you want to upgrade your gun to an ergonomic, kick ass, looking killing machine!
For AK users:
Shorty has rail setup for AK-47 for $60. You can get it once it re-stocks: Colors: Tan, OD Green.
And for SMG users there's a sweet $100 Holosight with a red/green option which has me sold once I earn $100