Asphalt millings - if they are resurfacing a road anywhere near you, stop and talk with the superintendent and tell them you are looking for some. Odds are they will give it to you for free because it reduces their cost to haul off.
Crushed concrete base - if you are in Houston, this stuff is everywhere and you can get it in a host of different sizes. Cherry Crushed Concrete, Southern Crushed Concrete and 2 or three other places have it. You can also get it stabilized with cement - really helps when you do the initial install and compaction, and it holds together pretty solid.
Caliche or crushed oyster shell work great, but will need periodic maintenance depending on traffic use, subsurface conditions and weather. Haven't seen much oyster shell being used in a while though, not sure how hard it will be to get a hold of. You can usually call dirt pits and get TXDOT road base as well - has lots of fines, and sometimes its really good, other times not so much. I've had good luck and bad luck with it.
Limestone, granite, etc. are also available, but will be expensive because they aren't native to here (gulf coast Texas, hill country is a different story).