Since the beginning of our craft, polymer clay artists developed a habit of looking at various household items as potential tools for polymer clay. That is why you can often see old incandescent light bulbs recommended in polymer clay projects as forms for convex pendants. This used to be a cheap and simple option, but with the spread of fluorescent bulbs, finding the old ones is getting more and more challenging.
The second option is also stemming from the same idea – trash to tools. If you cut out the bottom of a soda can, it can be used as a concave/convex form. The issue with this approach is that the obtained form is a bit more shallow (and it also has nasty sharp edges that you have to keep in mind all the time).
By now, there is a professional tool for this task is available from Polyform. It is called Sculpey Hollow Bead Maker. Nice, but a bit pricey. I also do not like that it has a few forms on the same base and feels kind of crowded to work with.
Aluminum Sphere Cake Molds. They come in a few sizes, cost less than a dollar per pair on eBay, and very easy to work with. Enjoy!
P.S. These sphere cake molds (3" in diameter) can be used for my projects described in the following tutorials: Sea Creatures, Openwork Pendants, Faux Metal Necklace.