Here's a collection of ideas from a previous post...
What to do when no one is around to throw with?
Don't have a throwing partner? No one to throw with? Here are 10 ways you can still practice your pitching.
Save up and buy a canvas catcher. It's a stand that has a square cut out in the canvas around the strike zone. It's great to have a bucket of balls and throw into the canvas catcher. That way, you can throw more pitches before you have to gather the balls.
Sting a large blanket between trees and throw into it. Or go to a baseball field, put one of those large metal trash barrels on home plate, and long toss from a bucket in center field.
Get a "ball catcher" such as the Franklin Jr. Pop-up Catcher. It folds up to a small circle for storage. It costs anywhere from $50 to $60. Or try hhe Easton Jr. Pitcher. This is more cumbersome to set up (still not bad, though) and you can change out the canvas backing and install a pitchback net. It costs about $80 at Sports Authority.
Use a 4-foot by 5-foot piece of plywood and lean it up against the fence. Tape a four-zone "target" to it. You can add some used carpet to the plywood to deaden the noise a bit.
Find a brick wall, outline a strike zone, color in the corners, and practice throwing trying to hit corners.
If you attend a church, you could use the gym, if it has one. Or try your local school gymnasium. You can make a portable pitching mound, and use it inside, throwing to a net ... it's a great help in the winter!
Get a net such as bird netting (you can get it out of most farm supply catalog) that is about 8' by 5' and hooking it to a doorway in your basement. It will take about 6 hooks (2 on top, 2 on each sides) and a heavy weight-cinderblock or bricks to anchor the bottom down. Then throw into it.
Leave the ball out and work on your mechanics in front of a mirror.
Line up 60 feet away from a soccer goal and try to hit the crossbar as many times as you can. You can also do this for long toss: Set up a bucket of balls about a 50 to 60 yards away and throw at it. After all the throws, sprint to the bucket and back, and do it again.
Put a ball in your hand and then a big sock over your arm and hand leaving 2 or 3 inches at the end and tape the sock on your wrist. You can do all your mechanical work and actually throw the ball with out a partner and being in your house. you cant work on location but you can work on feel and everything else.