Yes you can make your own clay molds. I have made many but my experience has been different than Jerry's.
I use Raku clay to make the mold and air dry it. This takes several days and I ususally slowly unwrap it from plastic wrap because I don't have a damp room.
Once it's dry I fire it to 1800F at 100F/hr to bisque it then just use kiln wash as a seporator for the glass. Most of the molds I have made have been flat with texture. So, the leaf mold you want should work out ok.
When making ceramic molds, remember to work
carefully with the clay, so that no air pocket are left.
Raku clay is good, it is made to tolerate rapid temperature shifts.
Other good materials are clay with chamotte (=grogg).
that improves resistance to thermal shock.
Paperclay (clay with cellulose fibers) is the easiest to
work with. It forgives most without cracking in the
bisquit fire. It will smell strange when you bisquit fire it,
and release some smoke. The resulting piece is more porous
and weights much less.
If your mold involves uneven thickness, the paperclay is best.
I often paint the first layer of kiln wash when the clay is
bone dry, before bisquit fire.
When your glass is cooling, it shrinks more than a ceramic mold. The mold shape must be such that the glass can
everywhere slide towards the middle. low undulation is acceptable, but no vertical steps in the wrong direction.
You'd be better to fire as high as your kiln allows (at least to 1500), rather than air dry. If you air dry only, you'll have trouble when you go to use the mold with glass. That's because most clays will shrink 10 to 20% when fired -- you have to make them shrink before you use them with glass.
Plus... if you bisque fire it it will be stronger.
An unfired clay body is extremely fragile because it hasn't been fired in order to harden the body. A little water, and you're back to clay. A little pressure or stress and it crumbles or cracks. If you don't want to fire it in your kiln, make your mold, air dry and take it to a ceramics place and have them bisque fire it.
Thank you for all your help and replies. I am going to get some Raku clay and try my hand at making some simple molds.
One more question...how do you store the unused clay? Should it be put in a fridge? or basement? I've looked for a message board on clay but can't seem to locate one.
cindabr wrote:One more question...how do you store the unused clay? Should it be put in a fridge? or basement? I've looked for a message board on clay but can't seem to locate one.
Wrap the clay in plastic -- a garbage bag works great. Double wrapping is even better. If it dries out, you can sprinkle water on it and bring it back to life.
I've made many glass tiles kiln carved with clay templates. I have some additional advice for you. Although I initially thought raku clay would be best, I'm now thinking of trying a high fired body. The reason is this, air is released from the clay up into the body of the glass and can cause large air bubbles. So to avoid this, I pierce my clay template all the way through with a thin tooth pick, pin tool or even needle. I do this in many places, not just areas of carving. This way the air can escape under the tile and not into your glass.
Also, and this may not matter or be obvious, but clay shrinks. Make your design about 10% larger than your desired result, depending on the clay body.
Molds can be made from clay, but I'd suggest you buy yourself a basic pottery book so that you have an understanding of how to form and fire clay. It's not very difficult, but some specific knowledge is required.
Clay can be fired from about 1900F to well over 2200F. You need to verify that your glass kiln can fire to these higher temperatures. Many kilns made specifically for glass don't fire higher than 1700F or 1800F.
I use "Low White Earthernware" clay which fires to Cone 04, about 1900F. When you read up on pottery, you'll find that firing temperatures are referred to as "cones" rather than exact firing temperatures. A number of kiln-formed glass books suggest firing pottery intended for glass molds to 2 cones levels lower than the stated firing range, so Cone 04 clay should be fired to Cone 06. Many pottery books have a table listing cones with equivalent temperatures.
In addition, clay is FUN! You can make very imaginative molds that the usual glass mold makers wouldn't dream of. Many basic pottery books also explain how to make plaster molds, which will be useful for glass casting and slumping.
I'm wanting to slump some leaf and flower shapes but I can't seem to find any molds for this. 
If I was to get some Raku clay, make a shape that I want....will air drying be enough to harden it?