Many of your local sporting goods stores often carry a good lineup of packs from various brands. However, you may find it difficult to find a design that fits into your budget. While many big-box department stores carry packs, they are often from brands that are not known for their quality.
Shopping online is one of your best options because you will have the option to select from the entire lineup of some of the bigger brands, such as Gregory and High Sienna. The good news here is that you really don't need to try a pack on before you buy because they are very easy to adjust and provide you with accurate measurements so you can find the perfect fit.