Is there an inexpensive generic dishwasher rack for Maytag MDB4000AWA. I need upper and lower rack.

These racks from Appliance Zone are the least expensive racks from the places that sell them.
If your racks are not too far gone might I suggest the White Rack Repair Kit it includes tine tips and touch up paint. You can buy a rust neutralizer at your local hardware store or below are some options for making a homemade neutralizer.

1. Sand any rust spots off your dishwasher rack as soon as you notice them. You may use fine sandpaper to do this with, and finish it off with steel wool.
2. Use a rust neutralizer or converter product to keep the rust from spreading any further. Rust neutralizers usually contain phosphoric or tannic acid, which halt the corrosive process so the metal part can be painted over.
3. Buy a bottle of dishwasher rack touch-up paint from a hardware store or online. This vinyl paint usually comes in a small bottle and can be painted on like nail polish--a little goes a long way, and different colors are available so you should be able to find one to match your rack.
4. Replace any rusted tines with rubber repair tips. These are often sold in kits along with the rack touch-up paint, and also come in different colors to match the paint scheme.

Cream of tartar, hydrogen  peroxide, and a product such as Borax or baking soda can be combined to create a homemade  rust remover paste. The Borax or baking soda can be replaced with other similar products because their main purpose is to add grit to the solution without causing damage to the surface when used. To make this rust remover, a teaspoon of cream of tarter and a quarter cup of Borax or baking soda should be mixed with enough hydrogen peroxide to make a thick paste.

The paste should be rubbed onto the rust spot and allowed to stand for 30 minutes. Then, the rust stain should be wiped away with a damp sponge. If the stain remains, the entire process should be repeated.

White vinegar is another rust remover commonly found in homes. When using white vinegar as a rust remover, it should be sprayed directly onto the rust stain and scrubbed in. Then, the area should be sprayed again with white vinegar and allowed to sit for twenty to thirty minutes. After being allowed to sit, the rust stain should come off easily when the vinegar is wiped away.

Yet another homemade rust remover solution is a cup of Borax mixed with the juice from one lemon. This mixture makes a rust remover paste that can be applied directly to the stain. It should sit for twenty to thirty minutes and then be scrubbed clean. It might take a couple applications for this rust remover to be successful, but this concoction is safe for many surfaces, as well as safe for pets and children.

I hope this helps.
Good Luck!