craigslist or ebay, but your going to be getting some old technology, if your talking a compound you need to get measured for you draw length and find out what poundage is comfortable for you to shoot consistently, not a lot of options for that kind of money, try to save up some more and you will have better luck...
If you are looking to just give it a try and not planning on hunting you could look at getting a Mathews Genesis. Them search here for all the different form threads and learn to shoot. If it goes well and you like it you can go from there.
What type of archery are you looking for? If it is recurve there are some youth bows/brandless overseas made bows for that much, but you won't be able to get any new compound bow for that price. You might be able to find some terrible or old second hand examples but you'll probably have a lot of problems with them.
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