"Would you be able to buy spherical or the other type of bushing you talked about and replace the polyurethane bushings that come with most lower and upper control arms?"
Depends upon the manufacturer. Steeda, for example, offers some of their control arms with urethane OR spherical bearings. You just have to designate which ones you want when ordering. Some others offer this, as well.
"Just wondering, why do most manufacturers sell uppers and lowers seperately?"
I don't know other than the fact that most beginning hobbiests/enthusiasts don't have the money to drop on an entire suspension project, but tend to do it in little steps. Besides that not everyone is running a stock 4-link. Some are running an aftermarket 3-link or 5-link suspension. Some folks have torque arm suspensions (like myself). Some, as I mentioned earlier, offer control arms with altered geometry, either uppers, or lowers.
You can't go wrong with Maximum Motorsports quality and service. Personally, I'd get the adjustable/weight jacker arms with the spherical bearings on the one end. They don't change the geometry, but are a far cry better than the stockers by allowing some ride height adjustment, a lot more strength, and less suspension bind.