You can find some on Ebay. Not terribly expensive and might get you going relatively quickly.

Here's a link. I made it tiny...
https://tinyurl.com/48t2tk5v

 

I Ordered brushes locally that are really close to correct size. I Need to file them down to a size. Seems to that no one stores any brushes here at all. No one repairs anything anymore so parts are rare.

 

The original poster says the problem is intermittent. Does the auxiliary pump fail that way? I last drove mine in freezing weather and the outlet temperature was freezing to match. Today I went for a drive in light jacket weather and the heater blasted me out of the car. I'm going to take my temp gun with me to chronicle the output, but there is no doubt about the difference. Last time, no heat at all. Engine up to temp, but chattering teeth at the driver's position. Today, heat works just fine.

 

From memory of other threads mentioning it. The heater pump is like a pond pump, the impeller is magnetically driven so I'd guess if there's some crap in there blocking it at times you could get intermittent issues.

 

There is the thing of tapping on the heater pump with a broom handle as this will jar the bruches loose to settle in to make better contact and run

but this would indicate time to replace them

the relay can be floating causing intermittent contact and you can junper between socket 3 and 5

The 2 power courses to the relay.................

Point 5 square is hot at all times

point 49 double hash circle is fuse # 3 / 10 amp engine compartment fuse box which is dependent on the ignition positive relay in the engine compartment fuse box to be closed , and your engine is running

The A / C control module is providing a ground on the red marked wire in the pic

the heater valve is provided power as the blue wire and goes on through to the shared ground point