Like many of the products in high demand on the market, you can find professional cameras on online sales sites such as Ebay, Mercado Libre or Amazon, where you will find not only various models, but also their descriptions, reviews and prices.
In addition, you can easily obtain these devices in almost any department stores' such as Liverpool, Sears, Palacio de Hierro and Fabricas de Francia, which are prepared to advise you on the subject and help you make the best decision through their specialised salespeople.
In certain supermarkets such as Walmart and Soriana, in the technology area, you will also find them, however, it is very likely that the variety of products is limited and that you will not be able to be properly advised on the subject.
Another viable option is to go to specific camera shops, such as Canon, Sony and Nikon distributors, which, although they are not easy to find, will give you the security that you are being correctly advised and that you will acquire a quality product.