Michaels is a great art supply store that I personally buy a lot of my stuff from. They have stuff for all ages, beginners and advances as well. Depending on what type of art your child wants to do, you will be able to find it there and at a reasonable price. They also hold classes on the weekends for those looking to learn new techniques.
If this 6 year old is really gifted and serious, buy him some 'big kid' stuff. Michaels, JoAnns is a good one. Any arts and crafts stores. Just don't buy a serious artist, no matter what age, finger paints and colored pencils. Get him a sketch book and some charcoal pencils if anything. Art is a beautiful thing.
Watercolors are cool for easy clean up and that's important. Also glue stixs, nice sissors, colored paper, crayons and markers, hole punchs and a larger easy zip pencile type bag to keep all this stuff in... Those are the basics
It depends on what s/he likes! Take the tyke to the art store once a month to pick something to try out. Encourage experimentation.
For each piece of art, (give praise of course) ask, "Do you like it? How can it be better?" "Does it look the way you want it to look?" Art is great for learning critical thinking - a tremendous tool for all development.