You can buy reindeer meat (poronliha) in any supermarket. The "Herkku" gourmet supermarket in the basement of the Stockmann department store (corner of Mannerheimintie and Aleksanterinkatu) is pretty epic and a good place to buy this or anything else Finnish; not the cheapest, mind you, but not hugely expensive either.
The canonical way to prepare it is sautéed reindeer (poronkäristys), served with mashed potatoes (preferably using Lappish potatoes, puikulaperunat) and lingonberry puree, and you'll find ready-to-cook bags of reindeer shavings in the freezer section. You can also find reindeer sausages, meatballs, sliced meats etc in the deli section, but most of these are (IMHO) overpriced and terrible. Reindeer steak I would steer clear from, the meat is quite dry so it's pretty hard to get this right.
The classical starter to this or any other Finnish meal is a shot of ice cold vodka, preferably Koskenkorva, straight from the freezer. Stick to beer during the meal though if you want to taste (or remember) anything.
Hyvää ruokahalua! But I do hope the guy you're trying to impress is not Finnish, since sautéed reindeer is considered the kind of thing served at school lunches, not a five-star dining.