A used Ford F-350 can make a great work truck. The F-350's were designed to run hard for a long time, so finding a used truck won't be a problem if you know what you're looking for.
It's not uncommon for an F-350 to have high mileage when you purchase it used. Since these trucks were designed as work trucks, many people will find that the used ones can have as much as 90,000 miles on them. As long as the truck has been maintained well and taken care of, high mileage shouldn't deter you from these vehicles.
You can often find great deals by looking for certified pre-owned Fords directly at the dealerships. A certified used Ford means that the car has been certified by the manufacturer to be in great condition. The certified truck will have to pass a thorough inspection to meet the quality standards set forth by Ford. It's not hard to find discounted used trucks at the used car lots, even if they aren't certified. As the dealers start to acquire too many trades and lease returns, they will start to reduce the prices and offer special promotions to move the used cars too. Keep an eye out for sales and events in your are.
Online resources are great to check as well. Auction sites like eBay can hook you up with sellers from all across the country. As long as the seller has good feedback and has proven themselves to be reliable, you are pretty safe using sites like this. Make sure you take necessary precautions when dealing with any kind of large purchase online. Only use payment methods that protect you, such as credit card transactions or PayPal. Wiring money or sending cash will leave you no recourse in the event that the purchase falls through.