Changing flange for avent breast pump

I'm 4 mths pp and find the breast pump already ineffective since 3rd month as I have to pump at work. Pump for very long only produce 10 to 20ml whereas if I latch for awhile I can get the same amount. Was advised to change flange but no idea whr to get the flange for my Philips Avent breast pump, any mummies know?