I find the best supply source is DIY home improvement and garden shops. Ask for flagstone pavers and river rock. Another excellent souce is a gravestone or countertop manufacturing facility. I get scraps from a countertop place locally for free!
I do not recommend using locally found rock from streams and creeks as there is such a high risk of bringing parasites and disease into your collection.
If you do decide to use found rocks, granite is your best choice. It is a very stable volcanic rock that is the least likely to have absorbed chemicals. Sandstone, quartzite and limestone are your next best bet. Avoid all sedimentary rock, especially shale, as these tend to have dangerous naturally occurring hydrocarbons in them.
The safest way to sanitize rocks is with a good long soak in a supersaturated solution of salt water for a week, followed by extensive rinsing and a good long drying period in direct sunlight
To make a super saturated solution, start with 2 liters of tapwater and bring it to a roiling boil in a nonstick coated cooking pot. Slowly stir in non-iodized tablesalt until no more salt will dissolve in the water using a plastic spoon or whisk. You will know you are doing this correctly if you get most of a 26oz(737g) canister of salt dissolved in to 2 liters. Let this solution cool to room temperature. While its cooling down, give all your rocks a good scrubbing with tapwater and a stiff bristled plastic brush (I use the wife's toothbrush for this, but don't tell her...). Once cooled transfer the solution to a suitable plastic container (to avoid being skinned alive by the wife
) and soak your rocks for a week or more.
As stated above, NEVER boil any type of rock, nor try to sterilize it with the heat of an oven...They can explode and cause serious injury due to rapidly escaping water vapor under high pressure.
