You should check these websites:
I am a member of bjs and have found them very competitive.
I considered purchasing a structured wiring cabinet from this seller but have not yet done so:
They offer a large selection of security cameras.
Once your select a system that you think meets your requirements, go to the manufactures website and find out how to call them via land line. Customer service should be able to advise you if you choice meets your needs. You will also learn how they treat customers by your contact with them. If their service sucks purchase from another vendor, if they provide helpful service you have already won half the battle, knowing that they will probably treat you the same way after you make your purchase.