yay for a new kitten!
I love the name Sadie. You'll have to post a few pictures when you get the chance.
Royal Canin is a very good dry food brand. They make Royal Canin Babycat (under 4 months) & Royal Canin 34 (Kitten) for 4-12 month-olds. I adopted my kitten, Chloe, at 3 months old and have fed her Nutro Natural Indoor Kitten since then. Nutro also makes Nutro Natural Complete Kitten & Nutro Max Kitten. Wellness makes a kitten dry food that I imagine would be very good, so does Chicken Soup.
Whatever you end up deciding upon, it's important to buy a little bag of the dry food she's on now so you can gradually switch her over to the new food over at least a period of 1 week. This is to avoid an upset tummy and diarrhea.
I would also feed wet food if you can and just supplement it with the dry food. You don't have to be careful switching wet foods, and I don't think she's eating any at the shelter anyway. Some good brands that make specific kitten formula wet foods: Nutro Natural, Nutro Max, Wellness, & Chicken Soup. Otherwise look for "suitable for all life stages" on the can/pouch, not "adult maintenance." Other good brands for wet food (not in any order): Natural Balance, Merrick, Blue Buffalo Spa, Avoderm, By Nature Organics, Evo, California Natural, Innova, Solid Gold, & Tiki Cat. (There are others, I just can't think off the top of my head). I try to vary what I feed my cats for wet food so they don't get too picky. You can feed kittens human baby food as a treat, make sure the ingredients just list chicken and water or meat and water and don't have added flavors like onion or garlic. I used to do that with Chloe.
I'm so excited for you, a new kitten is a wonderful thing! Thank you for rescuing a kitty!
ETA: You aren't going to find any of these brands at the grocery store, unless you have an awesome grocery store. Where I live, PetCo seems to have more variety than PetSmart. A small independent pet store is going to be your best bet. Some of the foods I listed are less expensive than others. Go with the highest quality dry food you can afford, you can be less picky about wet food. What you can do to save $ is leave a bowl of dry out all day for your kitten (kitten's can be free-fed, and you only have 1 so having other cats eat food that isn't their food won't be an issue). Then just feed wet 1x a day. Buy wet food in larger cans, it'll save money in the long run. Just get a lid cover so you can save the leftovers in the fridge. (Some cats are picky though and won't eat leftovers, I guess it's just like human kids, lol. You can try heating it up or sprinkling cheese on it if this is the case). For the price, I really like Nutro and also PetSmart makes a brand called Authority which is good.