Not less than an hour away....
There is a limestone quarry in Georgetown (just north of Round Rock, which is just North of Austin) where I filled up (literally, compressed the springs down to where the floor was scraping the ground just about) my minivan with limestone (probably about 300 - 450 pounds, rough estimate). Total charge was $25 (plus my time for getting it). the place was something like J&J Quarry, but if you went to the yellow pages section and searched around that area you could probably find 2 dozen quarries if you could find one.
Most of the rock was in my 300 G that went before I moved. I chipped the peices down (some raw peices weighted probably 50 pounds a peice) before I soaked them in some light (pool scrubbing) acid to wash the impurities off, then soaked them in bleach for 2 weeks, then in fresh water (changed daily) to get the bleach washed off.
You can see some pictures of the rocks here
http://www.wildfire-tech.com/~matt/tank/300g/gen_index.php?root=pictures&path=//2002-04-01_Rock/.