You can use anything referred to in the book series, as well as related food items that you think would have been appreciated in a Harry Potter context. Harry Potter party food is really easy to come up with. Just think of a food or drink you and your guests would like and come up with a creative Harry Potter name for it. Some suitable foods include:   Treacle tart. This has a Harry Potter feeling to it, as Hagrid serves it to Harry multiple times.  Roast potatoes. Baked, fried, and mashed are okay too - they have these at the start-of-term feast.  Pumpkin juice. You can use orange juice, or learn how to make pumpkin juice. This is a nice addition to any party. Chocolate frogs. Use chocolate in the shape of frogs. You can buy frog molds at cooking stores and online, or buy ready-made chocolate frogs from candy or sweet shops.  Pudding. Luna Lovegood says that you cannot have a party without pudding. Well, your party wouldn't be a party without pudding, either!  Butterbeer. There are loads of recipes, so look it up! Most people suggest microwaving a tiny bit of butter and some butterscotch syrup for a minute and then adding cream soda. Bertie Botts' every flavor beans. Jelly Belly beans come in almost every flavor and make a good substitute. Here are some additional ideas for things you can make at home fairly easily:  Strawberry acid pop drinks: All you need is strawberries and fizzy strawberry drink. Cut the strawberries up into little bits and tip into a glass, pour a reasonable amount of a fizzy strawberry drink into the glass and dilute. To make this into a dessert, simply add some strawberry ice cream. Spark wands: This is easy; all they are is bread sticks. The bread sticks are the wands, and if you want your"'wands" to shoot "sparks", then get ones with seeds on them. Crumpet faces: If you like crumpets and pizzas, you and your guests will be excited to get to one of these delicious morsels.  Get two crumpets and cover with cheese, olive oil, tomatoes, and olives if you like them. Make into faces or throw on. Heat in your oven for about 10 minutes. Eat and enjoy!

Summary:
Choose the party food. Make party food.