check "sporttogo.com"....8.5 inches to 15 feet!!
Cheers from here

Rick, could you give me a little more help. I went to the web site and there were just too many possibilities. Marie

If you have a "discount" (cheap-o) sporting goods store in your area (we have one called "Academy"), you can find Chinese knock-offs of the heavy-weight exercise balls up to about 40 inches. I have overinflated them up to almost 50 inches, just to see how far they would go with no problems. Got mine on sale for under $10 bucks each and they can be used over and over. I wrap some strips of hardware cloth (metal mesh) then build-up a fairly thin (half an inch?) base layer of fiber reinforced quick-set concrete leaving a 6 inch hole on top with the balls inflation stem centered in it. Let the first layer set well then deflate & remove the ball. Continue building up to the thickness you want. Note...changes in temperature will cause the ball to expand & contract so dont try to apply too much on the first pass. Leaving a few open areas on the surface during the first application seems to help a little. Remember, concrete warms up as it sets, but the open areas must allow the air to stabilize somewhat and prevent too much expansion that would otherwise "blow" the first layer off. Have fun...no, make that..."Have a ball".

The way that I have been making my spheres is just the opposite. I take the ball, make a plaster mold of the top half (to the equator). When that has set, I flip it upside down and make a plaster mold of the remaining half, digging a groove in the plaster at the equator. When the second half has set, I take it off and remove the ball. Then I clean up the edges. When I make a sphere, I line the insides of the two POP halves with plastic trash bags and build the hypertufa or concrete INSIDE the mold. Next day, I remove the two halves and make a little mix (with some admix this time) and glue up the two halves. The seam is always tricky. I have found the plastic saucers for plant pots are great for holding round shapes.

Melissa

Since you just want a difference of 2 inches more or less than the balls you have already used, I think you could either overinflate the 12 inch one til it is 14 inches or underinflate the 16 inch one til it is 14. While saying that, I will also warn than a "mushy" feeling degree of inflation is not good - you need a pretty firm ball to work with so maybe I am wrong about the above idea.
The balls I have bought fail to achieve the size they are labeled when I blow them up. That exercise ball I got that is supposed to go to 32 inches never got more than 26 for me. Hearing that someone got their 40 inch ball up to 50 shows me that I must not be trying hard enough when I inflate the things. I am afraid I will make them pop so I guess I don't put in the maximim air possible. The point of all this being that I think most balls must have a variety of sizes you can inflate them to and still be good for making spheres with concrete.
Laura

How interesting. When I need to do a casting like this I set the ball in a box of sand and pour the concrete or whatever into the ball which has become a mold and then after the filling has set up, the ball is cut away. If doing a two part pour like Milissa does, I use rebar to make sure there is a good connection. To reuse the ball, I tape the thing together with duct tape and set it in the sand box with only the pour hole exposed. The sand prevents the ball falling apart most of the time. For a ligher weight finished product, I insert a styrofoam form with thin wood or metal pegs to keep it centered. You could also make pegs from concrete to hold the styrofoam in place and keep the walls uniform. The wood pegs can be drilled out and plugged with a touch of concrete for finishing while the material is uncured. Metal pegs can make a good attachment point for attaching ornamentation if they protrude 1/2 inch or so. Sandy

Sounds like we're bouncing between solid and hollow spheres!
Sorry,couldn't resist-it must be the eggnog talking!!
Cheers from here