I usually shop at J C Penney for new clothes (because I have a store card and work in an office with a dress code), Target (affordable and I like their tops), or Kohls if someone gives me a gift card (sometimes for Mother's Day, Christmas, or my birthday). Check out local consignment shops, too. I have been finding some cute and affordable maternity items on ebay, so that is also an option for gently used clothing on a budget. (You could also sell things that you have "undergrown" that are nice to fund your new wardrobe.) I am 27 and find that the Misses departments usually have clothing that is appropriate and stylish at the same time. If you find a few fits and styles that are really flattering, stick with those. If you are breastfeeding, though, wait a bit before buying too many shirts. I dropped too many sizes in the year after I had my younger son and it seemed as soon as I replaced everything I had to do it all over again. (And then when I weaned, some things were still too baggy.)
Just to try not to feel frumpy, I recommend finding at least one or two pairs of jeans you love (you can get them cheap--I also LOVE jeans from Victoria's Secret. Depending on your figure they are very flattering and can get to clearance prices around $20). Keep a few basic t-shirts that are also slightly fitted (not trashy) and cute (Target is great for this because you can get them on sale for anywhere from $4 to $10 and they are fun and relaxed). Get a pair or two of flats for weekends and errands at least. I avoid wearing my husband's clothes because that will instantly make me feel nasty. So, stick to a few basics that fit your budget and gradually expand as you can afford it.