You can purchase foam and batting at the fabric store ,I would get a fabric that i used outdoor ,its more resistant and easy to wipe clean

You can find various depths of foam, as well as fabric, at your local fabric or fabric/craft store. Find someone local (re-upholsterer, for example) so you have some recourse if the item doesn't turn out as you wanted.

Take the measurements and head to a fabric store. I would suggest using indoor/outdoor fabric and they can offer batting and foam quality options for what your needs are - much less expensive than buying from the upholsterer - which could be double. They may also have names of local seamtresses, who would be happy to do this small job (or ask friends for a recommendation) - again less expensive than an upholsterer - just be sure to ask if they have done something like this before. My mother was a wonderful seamstress (sewed my wedding dresses, reupholstered furniture, etc), so I'm comfortable recommending this route for this easy job..it's not a couch, which I would recommend an upholsterer for.

If you have having custom made get the fabric and foam from them. It is usually better quality than purchasing from a Joann's type store. Plus then you have recourse if there is a problem like sooz001 says

Any suggestions of where I could shop for fabric online?

I wouldn't advise shopping online unless you can get a sample of the fabric first.

Google is your friend when it comes to finding stuff. Just google what you're looking for, and you should find dozens of sites.

Check in here

http://www.decorativefabricsdirect.com/Outdoor_Upholstery_Fabric_s/682.htm

You might check out sites for people that make cushions for piano benches.

I would go to someplace like Calico Corners where there are very
knowledgable and helpful experienced salespeople. Plus a list of (unconnected) folks who make drapes, slip covers and cushions,etc. I have had only good experiences.
I also had a bench cushion made by a woman who had just put her sign in her window "slipcovers". I was able to see her work.
She estimated how much fabric I needed and did an excellent job.
Too much so, I guess, as the next time I wanted something done there was a months long waiting list.......