Well for general price trends you could look
here.
Contrary to what one might think, in recent years the prices have been lowest in January.
For the absolute low prices you've missed the
Schiff so to speak. However its not that bad currently & if you need it then...
As for where to buy it - look in your
Gelbe Seiten and phone a few up - the price per litre will depend on the quantity. We jointly order with our neighbour to get a lower price.