I don't know if you will find a sonar for that price. The cheapest are $150 and they go up from there. You may be able to find a good used one on ebay or fishing forums somewhere. There are lots of models capable in scanning for fish and structure in the basic package. You need to decide if you want detailed images (HD) or just something that shows dots or fish on the finder.
I have a 16' jon boat and I use a Lowrance Elite 4 DSI amd it ran me $350. You can find them cheap used. Just do some searching for used ones and get info from the sellers.