Lots of swimming pool owners are do-it-yourselfers and, as such, they want to buy the parts they need to maintain their pool equipment and install them on their own.  Some go to the local pool supply stores where they can get guidance from the pool experts in the store while others try to buy what they need online.  When shopping for pool equipment online it is important to be prepared and get all the critical information off your existing pool equipment such as the pool filter manufacturer/model, filter cartridge part number, pool pump manufacturer/model, and pump motor part number.

This information is the most critical since it is from the equipment that is at the heart of your swimming pool:  filter, pump, and pump motor.

When searching for pool filter replacement cartridges or D.E. grids:
Your pool filter manufacturer and model should be on the outside of the canister somewhere.  The major manufacturers are Hayward, Sta-RitePentairJandyJacuzzi, Harmsco, and Waterway.  If you are looking for replacement parts or a replacement cartridge filter or DE filter grids you can either search for them by the manufacturer/model or the specific part number of the replacement part you need.  For example, in your preferred search engine such as Google you could enter the search words,  "Hayward C1200" or Hayward CX1200RE.  The search results should provide plenty of places online for you to shop.  

One bit of advice is to check the product details of the item you plan on purchasing to determine the manufacturer of the product.  Many resellers online advertise the original parts product number but actually sell a much cheaper, lower quality knock-off in it's place.  For example, they will charge a price comparable for an original Hayward filter cartridge, but actually deliver to you a knock-off of lesser quality that is the same size.  Many of these cartridges do NOT provide the same filtration capabilities as the originals nor do they have the same durability, meaning they will have to be replaced much sooner than normal.

Not all after-market filter cartridges are bad.  In my experience, there are two top quality after-market filter cartridge makers:  Unicel pool filters, and Pleatco pool filters.  Both of these companies are in the U.S. and they make their products in the U.S. as well.  Further, they design their products to the specifications of the original manufacturer.  The main difference between the two is that Pleatco uses a different filter fabric that is a pharmaceutical grade materia.  It provides excellent filtration and water flow, however, it doesn't seem to last quite as long as the Unicel or original manufacturer cartridges. 

I hope you find this information helpful.

Gary
All Pool Filters 4 Less
www.allpoolfilters4less.com