Your parents got divorced when you were in your early teens, and your dad remained a single man until recently. Sure, he had weird dates with a parade of strange women wearing funky jewelry and speaking to you in high-pitched, overly friendly tones ("Hi honey! How ARE you? I'm so excited to finally meet you!"), but they never stuck around for long.
But shit changed a few years ago. Your dad met a nice, age-appropriate lady with kids of her own. He finally had someone to have dinner with and go on trips with. They got married in a modest backyard ceremony. She wore a tasteful cream-colored pantsuit. And because you're a fully-grown, semi-reasonable adult, you either genuinely like her or you're ambivalent about her. None of the anger and mutual hostility that your friends had when they got a stepmother at 12 years old exists with the two of you. If your dad's happy, you're happy. And if someone's around him all the time, you don't have to worry about him so much.
But all of these warm, fuzzy feelings (or just general apathy) don't mean that you're close. Step-parents are always hard to shop for, but they're ESPECIALLY hard to shop for if you've just met this person a year or two ago. You have more interaction with the barista at your local coffee shop than you do with a person that has the word "mother" or "father" in their titles.
You try to pry advice on what to buy out of your father, but as always, he's useless with this kind of stuff. Just try to find out the basics. Does she have a dog? What is her profession? Does she like wine? Who doesn't like sweaters, amiright? Armed with these basics facts about a virtual stranger who you spend a few times a year with, you'll be able to figure something out for her. Either that, or get her a gift card to the Olive Garden. Stepmoms love the Olive Garden. Because breadsticks.