Hi and welcome to the wonderful world of quilting!
You will soon discover that your choice of fabric is as unlimited as you want it to be. Between online shops, local quilt shops and JoAnns the selection is endless. I have been told by professional quilters to use ONLY 100% COTTON FABRIC. Many long arm quilters will not use anything else. However, there are pros and cons to using anything other than 100% cotton. Seams are difficult to stay put and the fabric seems to ravel causing issues with sizing once you cut the pieces. Try different fabrics to see what you prefer. A wall hanging that is made up of blends may be dry cleaned rather than thrown in the washer.
WalMart has 100% cotton fabric of different weights so stay away from the really thin - see-thru kind because it will not hold up during washing or sewing. You can actually feel how thin some of it is and with all the hard work and long hours you will put into your masterpiece you want it to hold up.
The best places are the quilt shops. Their advice and experience is as valuable as the fabric they sell. When I took my quilting class at my local quilt shop, the staff helped me find the perfect palate for my blocks. I was happy with the selection and didn't have to drive to Timbuctoo for it. A little on the pricey side, but it balanced out saving on gas.
Whats even more fun is making a quilt with different style blocks in different colors. This will give you an idea of color, style and difficulty levels to work out.
Good luck and Happy Quilting