Here are some deals from RockAuto. I subscribe to the RSS feed and when there are closeouts they alert me. A recent topic about improving braking by going to a bigger master reminded me about this. Note the bore size and if it's bigger than your's then you could get an improvement.
Part Type: Master Cylinder
* Manufacturer: BENDIX
* Part No: R11738
* Notes: With power brakes; Rear drum brake 9 1/2" x 2"
Closeout Price: $10.03
Only 10 Remaining!
* Manufacturer: WAGNER
* Part No: R101253
* Notes: w/Power Brakes
Closeout Price: $10.15
Only 1 Remaining!
* Manufacturer: RAYBESTOS
* Part No: MC39016
* Notes: Power Brakes
Regular Price: $25.79
Closeout Price: $13.60
SAVE 47%
Only 3 Remaining!
* Manufacturer: AUTOSPECIALTY / KELSEY-HAYES
* Part No: M82003
* Notes: 24MM - w/Power Brakes
Closeout Price: $17.26
Only 5 Remaining!
* Manufacturer: WAGNER
* Part No: MC101253
* Notes: w/Vac. Boost; 24mm Bore
Regular Price: $34.89
Closeout Price: $18.69
SAVE 46%
Only 1 Remaining!
Part Type: Master Cylinder
* Manufacturer: BENDIX
* Part No: R11738
* Notes: With power brakes; Rear drum brake 9 1/2" x 2"
Closeout Price: $10.03
Only 10 Remaining!
* Manufacturer: WAGNER
* Part No: R101253
* Notes: w/Power Brakes
Closeout Price: $10.15
Only 1 Remaining!
* Manufacturer: RAYBESTOS
* Part No: MC39016
* Notes: Power Brakes
Regular Price: $25.79
Closeout Price: $13.60
SAVE 47%
Only 3 Remaining!
* Manufacturer: AUTOSPECIALTY / KELSEY-HAYES
* Part No: M82003
* Notes: 24MM - w/Power Brakes
Closeout Price: $17.26
Only 5 Remaining!
* Manufacturer: WAGNER
* Part No: MC101253
* Notes: w/Vac. Boost; 24mm Bore
Regular Price: $34.89
Closeout Price: $18.69
SAVE 46%
Only 1 Remaining!