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There are break-your-own geode kits for sale online. You can find them either individually at about $2 apiece or in boxes/buckets of 10 for about $15-$10.
Absolutely check with the Rock Hounds in your area!!! It's likely cheaper for you to get a truckload and crack/epoxy them yourselves (not kidding about this!)... Otherwise, watch the auction circuits - It seems like every now and then there's a rock hound retiring and selling all of their rocks. OR, another option would be to go on a vacation somewhere that you can collect your own (Lake Superior Shoreline for example has amethyst all over, and rock hounds are going to know where the best places to go. DIY is going to save you a TON of money on this one!!!
Or, you could change your approach - you could order bags of finished rocks by the pound (like they sell in the gift shops) and just use them as sort of confetti stones around a pillar candle on a mirror..