Most fish stores will have it; Neptune sells packets and any online mail-order shop will have it. If you will be using the probe in an aquarium you want to get 7 and 10 and if you will be using it for CA reactor get 4-7.
X2 on your LFS. The one here in Baton Rouge stocks them for $1.99 each - 4, 7 & 10. Buy a couple and keep them handy for re-calibration when you need to.
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