In US, the best way to approach it is get something on ebay.
Apple products will be easy - you can get iphone 3g/3gs for around $50-$60 on ebay.
Android is a bit more complicated, as there are many different models. Specs wise, I would advise something around Samsung Galaxy S (the first one, also known as SGS) if people (your potential clients) are getting new contracts, almost everybody will have specs beyond the SGS. Even the entry models (that are not very popular in US, but are quite popular in Europe and middle east) will have similar specs to the SGS. So if it will run well on SGS - it will run well on almost anything used nowadays ;)
For Windows 8 phone and Blackberry, at this time there isn't a good way to do it. If you are lucky, you might find nokia lumia on ebay or pre-owned somewhere.