Definitely Spazz over the kids at Vanguard.

Sector 9 Airsoft also does upgrades. (602) 464-4487

Would help to know what weapon platform you're looking for.

Pistol? Rifle? RIS? Vertical foregrip? M4? AK? 1911?

Did you check the Airsoft Items For Sale section?

http://www.airsoftarizona.com/index.php?topic=36344.0

Added
Sector 9 Airsoft
Fear Farm
Allied Surplus

Updated
Gearbox
Command Post
Fightertown

So lets say i swap out the spring, what would it take to build an upper reciever?

You'd need:

-Matching upper receiver
-Miscellaneous upper parts (dust cover, charging handle, etc.)
-Outer barrel & muzzle device
-Front iron sight
-Handguard or rail kit
-Inner barrel
-Hop up

If you had some variable velocity system like polar star or a split gearbox, then it might be worth your time.

I'd say you're better off leaving your M4 as a field gun and getting a separate gun for CQB.

If you already have magazines and a load-bearing rig for your M4, you could get a short barrel M4 like this, which is roughly the same size as an MP5.

Mine ripped on me after about a year, but it was a great rig while it lasted.

What on earth ripped that caused the vest to be completely unusable?

I had both the shoulder buckles get shot out on mine, but I never used them anyways so I zip-tied them together. Mine is really beat up after 3 years but I'm still able to use it without problems.

I was just talking to Brian about this. Basically they're trying to promote the orange tip because stupid kids are taking off the orange tip and doing stupid shit in public view.