Treadmill Doctor find that the reason for the problem, 85% of the time, is that the deck and/or the belt are worn. This is in their repair staff's experience. They say that the second cause (8% of the time) could be the walking belt and/or motor are too tight. This makes a lot of sense because if the belt or motor is tight and you put a heavy weight on it (your body) then it's going to slow down.
They say only 5% of the time have they have diagnosed the problem and found that it's the brushes that need changing. That's very small. The last cause (2%) is the controller dropping output. If you're confident it's the brushes and plan to change them yourself then my advice would be for you to approach your manufacturer and buy them there.
If the manufacturer don't sell them or you want to call someone out to diagnose and fix the problem then Treadmill Doctor can do both for you. They've been online since 1999 and have many good reviews on the internet. They've always answered any email question I've had with great detail and speed. They can help service or repair all kinds of exercise equipment and have good advice on their site.