In some cases, convenience is the most important thing in choosing soap-making supplies. An easy-read thermometer from the grocery store is going to be the same easy-read thermometer from the soap supply specialist. A lot of the equipment you will need for soap casting can be found in your average big-box department store or even the supermarket. When you're shopping for soap-casting equipment, check out your local stores to find the best value.
The casting soap artisan's best friend is a stainless steel double boiler with a lid. You can improvise a double boiler by using a metal bowl over a saucepan. You can even do some projects using heat-safe heavy glass measuring cups. However, a good double boiler set will make your life much easier and your soap much better.
You will need an instant-read thermometer. There are many instances in soap casting where accurate measure of temperature is essential. Get two, as you may need to measure the temperatures of two containers at the same time.
You can still use your double boiler for food after you've used it for soap casting. Just be sure it's thoroughly clean. Even a minuscule bit of soap can make food taste funny. It's not enough to make you sick; it's just not at all delicious.
As in lye soapmaking, you'll need a scale. Since the chemistry is already done for you, you don't need to have as accurate a scale as you do for lye soapmaking. You should, however, get the best scale you can afford, both for accuracy and because it is likely you will need it when you make cold-and hot-process soap.
Rubbing alcohol in a spray bottle is a simple and essential tool. A spritz of alcohol will eliminate the bubbles that rise to the surface of the soap after pouring. It will also help you prepare the surface of cooled soap and embeddable objects to adhere to a second pour.
If you also make transparent soap using the hot-process method, you'll have ethanol on hand, and that will work perfectly in all soap-casting applications in a pinch, but rubbing alcohol is much less expensive and it's easy to get.