I would go to several furniture stores and actually look to see what they have. Check the sturdiness for yourself and determine what's the most practical and suitable for you're his room and your purse. Look for quality first, then price.
The best option is k-mart or walmart but if you can spend some time looking for it, look for ads about the moving sale, will get some really good deals. Craigslist is also the best option, quality furniture for less but make sure you have a thourogh check before paying for it. Thrift store also works out.
You can go to places like Ikea that have furniture that is durable yet stylish and affordable. You can also purchase furniture from thrift stores or from classifieds in your local paper if you don't have that much money to spend. Walmart can be good but look at reviews online first because some furniture isn't that good.