You absolutely should visit www.hipmonkey.com - a retail website that 100% of profits go to assisting families who need proper car seats that can't afford them. And, they price match to anything you can find on the web.
Their customer service is fantastic, and they are trained in the very best safety and product information. A woman named Angela Lam helped me find a carseat for my 11 month old 21.5 pound child. One that he will be in for many years. They can help with size, features, etc. However, the most important feature is True Side Impact. It is a must. I went with the Recaro Signo, and am so happy. I feel that if an accident were to happen, he would absolutely be safe. Find the contact us number on their website and give them a call or email. They will steer you in the right direction, 100%.
Also - to the people who still consider boosters - they are not as safe as you think. Keeping your child in a 5-point harness as long as possible is imperative. Also, they have now changed the recommended time rear-facing to 3-years. Hard to do, but so much safer rear-facing. Check out the video about Kyle Miller - and spread the word. It is heartbreaking, but at least their loss can help us learn how to keep our children safe. http://www.youtube.com/watch?v=azgBhZfcqaQ