First of all, most areas have oil distributors of some type, such as home heating oil companines. Call them and ask if they sell off-road diesel, many do, or will be able to tell you where you may purchase it.
Stations with off-road seem less common in some areas, but you may be able to call the different brand name companies and ask where you may purchase it at a station.
I personally buy mine from a fuel oil distributor locally in 55 gal drums that I take over there on pallets. 100 gallons or more saves me about $.50/gal.
As for using in winter, in most colder states, diesel fuel is mixed for a cold environment with #1, kerosene to reduce the instance of gelling. When this is done has more to do with the area and when colder weather tends to set in. Just ask the distributor about treatment. If it's not treated, there are plenty of additives one can purchase to add to the tank to prevent gelling. Power Service and Stanidyne (sp) come to mind. Make sure you purchase the product that says anti-gelling as they have several different prodcuts.
John