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The National Biodiesel Board in the US maintains a list of biodiesel suppliers. They also track the biodiesel price and keep statistics of biodiesel prices at the pump. The list is a great source of biodiesel locations and any biodiesel buyers is the US should access the National Biodiesel Board's web site.
There is also a great interactive map of US fuel stations selling biodiesel at http://www.altfuelprices.com/?fuel_type=64
The EU also has a biodiesel board, the European Biodiesel Board or EBB. Their website is http://www.ebb-eu.org/ and they have a list of members on their website.
In other countries it is not quite so simple. The best bet on finding a supplier in your area is via Google. Google has become wonderfully regionalized. So say I live in Tunisia, if I Google 'supplier biodiesel' I will get a list in which Tunisian suppliers are given preference.
Remember that when you look for a supplier you should look for a business that produces fuel to an accepted standard. As biofuel suppliers they want to sell fuel so they should not be that hard to find.
Google's regionalization also applies to states and areas within states and cities and towns. So a quick Google should turn up some useful information for you in your area.
The other way to find suppliers is to find biodiesel machinery manufacturers in your country and ask them if they have sold any units in your area. Again Google is your friend!