Sleep sacks, wombie sacks, halo sacks, etc. are great for not knowing what size your kiddo will be. Everything was going splendidly well with my pregnancy until all of the sudden my water broke at 35 weeks. The girls were both around 4 1/2 lbs, and they not only wore premie clothes, but the clothes Id bought them in preemie size were too big. Some preemie clothes are really newborn clothes, and some newborn clothes are the size for a 10 lb baby. My girls are 8 weeks, and they now fit in to newborn clothes that are for 7-10 lbs (they are almost 8 lbs).
We still have some preemie clothes that they can wear (at 8 lbs, they obviously werent true preemie clothes even though they were ordered from a special preemie website).
IME, the best thing to do would be to call around and make a list of stores in your area that sell preemie clothes. I wish Id done that. Then, you can send someone to get a few things. Have them take a pic with a smartphone and send it to you. Otherwise, I suggest buying some preemie clothes from Carters website online if you get preemies, because they have cheap expedited shipping. Go ahead and pick them out and put them in your cart. If your babies come early, click "checkout" the day they are born have them shipped to you.