They are legal, for now.
Kind of tough to shoot without a pistol grip, but it can be done.
Keep the grip for when you go to Nevada to shoot.
The upper recievers are the registered part. Uppers are available at gunshows, but awfully expensive that way. Find an FFL in your area and see if they will do the paperwork for you. As long as their fee isn't outrageous, then you order from one of the many sources in Shotgun News. Some companies will only deal with FFLs and some will accept orders from the general public. DSA, probably the best around, will not, at this time, sell complete rifles or upper recievers to CA residents. Entreprise is a CA company and will sell rifles or recievers to CA residents, but without pistol grips. I think Arizona Response Systems has a policy of selling to CA residents as long as CA laws are complied with (again, no pistol grip). AZEX is another good source, with a good reputation but I don't know much about them.
For really good info go to www.fnfal.com.
Their site is having technical problems at this time but should be up soon (I hope!).
If you find a friendly FFL, I would recommend buying a reciever from Dealer Warehouse in Modesto, CA (209)545-2800 Imbel gear logo upper reciever for 218.50. They will deal directly with you and ship to your FFL. Then get a kit and some U.S. parts to make it legal. This method can be pricey but you learn the weapon from the inside out.