There are many European restaurants in Sri Lanka or just restaurants and cafes that serve European food, you shouldn't have any problems with finding one. You can order pizza, pasta and other dishes you're used to back at home.
The basics of Sri Lankan cousine are rice, vegetables and seafood. If you ask the waiter to make your food less spicy, you might not even notice any difference from what you usually eat!
Most hotels also serve "continetal" breakfast or dinner which basically means European-type of food like scrambled eggs or omelette.
Grocery stores and convenience shops have all types of food: you can buy any vegetables, rice, fruit, butter, bread, tea, milk, cheese, etc, without trouble and cook something yourself. For fresh fuit, vegetables, and seafood it's better to head to a local market where local farmers and fishermen sell their goods.