I wouldn't use peat. It takes up calcium and magnesium and is not very stable. It will dip your pH to the 4 level in a heart beat. I've been using rinsed coco coir and vermiculite with river rock in the bottom of the containers in my ebb and flow. In my drip type system I used coco "chunk", which I reuse after a little peroxide disinfection. In my potted plants I've been using finely sifted compost, vermiculite, and coco coir in equal parts with awesome results.

Thanks joe,
I would use the coco chips also myself, in fact that's what the plants are in right now. The only problem is that I can't get it anywhere locally, and it cost a lot to ship, $10 for 2 cubic feet of coco chips, and $10 for the shipping. I can get vermiculite locally, but not much better at $19.95 for 2 cubic feet. And perlite is about the same price.

I do have some coco chips left, but not enough, I need about 10 gallons worth, I have about 1/3 of what I need. Like you I also use rock (I cleaned and sanitized) for about the bottom 1/3 of the containers. It does a good job at allowing good drainage, as well as adding weight to the containers to keep them in place, also it takes up some space so I don't need as much growing medium.

I was planing to use a mix of peat moss, coco chips and vermiculite (I already have a small package of vermiculite) in this system, but I would rather not have calcium and magnesium problems, and it simply wont work if it's going to constantly drop the pH. Any ideas on something that I would be able to get locally other than vermiculite or perlite.

What type of system is this one?

It will be a drip system, probably in 3, 5 gallon buckets. bottom 1/3 of the buckets will be rock.