Pangeareptile.com is the cheapest you will get with shipping. Even cheaper than the revered reptilebasics.com which is a close 2nd.
You should run either 6" along the side and across the bottom then snake it down through the shelf and side (may need to loosen screws) which is why i suggest evenly placed screws. Or you can run an 11/12" single sheet per shelf and then wire them in serial which means you only need a few pieces.
For your setup you can run SpyderElectronics or VE but if you are still looking to budget, I know plenty of people running hydrofarms. Amazon has it the cheapest.