This is the stuff that you want to use. It is a medical grade acrylic. It is a bit pricey (aprox. $60.00/ qt.) But it is the best that I have found. There are other "kits" out there but none will give you the results that this will. You will also need a mold with smooth sides to do the casting in. You can buy some mold release at the hardware store. If they do not stock mold release they will be able to order you some.
Click on the link below and scroll down about 1/4 of the page and look for a pack of money encased in the acrylic. There is a shopping cart link there to do the purchase.
I hope this has answered your question. If it has then please be sure to click the accept button.
Best regards; The Home Smithy