We'll send you a couple pieces of chert, agate, quarts, novaculite, or jasper to use with your Steel Striker (we've got a big bin of various stone). Use caution when handling them as some may be really sharp. But this is what you want. A sharp stone scrapes the best sparks from the steel.
Finding "Flint" in your area:
Many types of stone will throw sparks from our strikers. A general term for such stone is Chert or Flint.
Look for stone that has a crystal or ceramic look and feel to it. It should fracture into flakes when struck. Some places have suitable stone in abundance where others will have very little. Carry your Steel Striker with you when out and about and experiment with various stones found in your area. You may be surprised by what you find.