Collecting it from your dive site
sounds like a good idea (save some $ right?) but in fact can have several issues. One, check your local laws to ensure you are allowed to do that. Two, it could contain some bad stuff. For sure don't collect from the waterline area. That's bad news. If you want live sand you need to take it from a distance/depth so as to avoid some pollution. If you take it from above the waterline, remember people spill all kinds of liquid while playing on the beach. You will need to wash it really well before use, thus it will not be "live sand"
Thirty years ago my dive instructor would collect water from the San Diego beaches when we dove there. We would swim out with jugs and take water at least a 100 feet from shore, his tank always had issues with water quality.
Your decision on how to proceed but is it really any cost savings if right off the bat you are dealing with bad water issues?