The Clay Shop
Mud Tools! I like a couple of yellow and green with a variety of corner shapes. http://www.theceramicshop.com/store/category/11/203/Ribs/Rubber/
Brushes-these are nice to have, any selection will do
http://www.theceramicshop.com/store/product/5076/Brush-Set-Talkon/
A rubber finger tool! I can't make a mold or build anything without it.
http://www.theceramicshop.com/store/product/1512/Wipe-Out-Tool%2C-Rubber-Shaper/
If you still need the basics- this is great:
http://www.theceramicshop.com/store/product/1690/Ceramic-Student-Tool-Kit/
Plaster
http://www.theceramicshop.com/store/product/395/Pottery-Plaster-%231/
At any store or from your home
Murphy's Oil Soap
A collection of small containers - Chinese Soup type
A towl
An Apron
A large sponge for clean up (its always good to have your own.
This stuff is at Home Depot or Lowes in the painting department-
A couple of buckets for mixing and to keep slip in
http://www.homedepot.com/h_d1/N-5yc1v/R-202264039/h_d2/ProductDisplay?langId=-1&storeId=10051&catalogId=10053
You also might want a variety of wet/dry sand paper:
Silicon Carbide Drywall Sanding Sheets
400-600 grit
and drywall screens - I like about 200 grit
Resperator
Blue Painting tape
Also in the Home Depot roofing department
You may want to have your own flashing... or you can share.