I don't go outside of my established sketch card collections all that often, but every so often I will pick something up that isn't on my usual list. Usually it's because someone is running a sale or I am chasing an artist who hasn't done artwork featuring my favorite characters (or they have, but I missed out on it).
I haven't seen any artwork from Mike Hartigan featuring Power Girl or Leia, but I really like his art style. Last year I got one of his Captain America sketches from Upper Deck's Avengers: Age of Ultron set. I think I also got the pictured Quicksilver sketch from the same set late last year, but it's been sitting in my post queue for a while. I don't like the Quicksilver character very much, but I thought this sketch of the speedster was too good to pass up. He's got an awesome cartoony style, and he uses a lot of bright colors that really pop. It looks like he's having a sale in his webstore right now. His normal prices are really good, and his sale prices are even better. He also has commission prices listed at the top of his Facebook page. I'll definitely be looking for more of his work in the future, and maybe I'll even go the commission route! I've been commissioning quite a few sketch cards from artists recently, and it's pretty exciting to get exactly what you wanted in the artist's signature style.