You say you have two of these which suggests that you do a lot of drilling - needing to have two - and presumably you use them to do different work - not needing to change the tool.
You suggest but do not say specifically that only one of them has developed slowness.
Brush wear might be a factor and so might environment - your work might create dust contamination that's getting inside - possibly gumming up the bearings or the commutator.
But....
In the UK Amazon sell brushes for this model £1.69 + £3.99 delivery.
Good luck...
Peccavi
April 2013