I have gotten several types of stone locally from an aggregate company.
Step one - Go into office
. Not a phone call.
Step two - Politely inquire about small purchases and mention what you are using it for
Step three - Likely pay $0-$10 depending on amount.
Step four - Choose what you fancy and collect it.
Step five - Profit....
Most people when they learn what you are planning to use this stuff for will gladly help you (some want to see pictures when you mention live planted tanks so bring that camera phone
). Most of what you want will be sold by the ton. So you will likely be paying $5 or nothing at all for a "sample".