Planners expect the additional aircraft to cost some 2.5 billion euros ($2.9 billion) according to confidential documents prepared for parliament's budget committee, the source told Reuters.
Germany has already ordered 35 of the US-made jets to replace its fleet of 85 ageing Tornado fighter jets, which are set to be decommissioned.
The F-35 aircraft are designated to succeed the Tornados in their role of carrying US nuclear bombs stored in Germany in the event of a conflict.
Berlin previously denied plans to acquire an additional 15 F-35 fighter jets in July, but a significantly expanded defense budget has since created more scope for weapons purchases.
At the time, a military source told Reuters that the idea of acquiring 15 more F-35 jets had been part of earlier discussions.
Any decision to purchase more F-35 aircraft could spark fresh tensions between Germany and France who have been at odds over their struggling fighter jet project FCAS, or SCAF in French.