It's not easy, as discussed earlier.
However, I would say, google for the three or four sites that provide information on lpg autogas suppliers, then ring any that are relevant to you, and check whether they do supply autogas, or if they even exist.
Then, check calor and flogas websites, and again, check any of interest by ringing them.
Check out lpgforum and post on there asking for info, it's not the busiest of sites, but you never know your luck.
Lastly, ask other users whenever you meet them, especially if they have a taxi plate, they always know the best places for anything.