Answer:
It really depends on how big of an event you plan to cater for. If you are doing small events, then you just need the basics: chafing dishes, serving utensils, serving trays, insulated bags or carriers for transport, dinnerware and flatware (possible the nice, disposable kind), and linens. If you are doing large scale events, then you might want to consider getting a catering van, hiring a small staff, and purchasing larger items such as a chocolate fountain, margarita machine, or a your own tables and chairs (if they aren't supplied by the venue.
Try these sites for more info:
http://www.williamssonoma.com/
http://www.missionrs.com/catering-supplies.html
http://www.business.com/directory/food_and_beverage/restaurants_and_foodservice/equipment_and_supplies/smallwares/linens_and_table_coverings/