How can I choose the resistors for a push-pull class AB amplifier (diode biasing) to get a certain bias current?
The problem is I cannot find beta (current gain) from the data sheet
  to deduce Ib and the current in the resistors. the date sheet provide
  beta for maximum of Vce=1V so I am stuck.


At low Collector current the current gain doesn't vary much above saturation, so you can take the value at 1 V and it should be close enough. The Base-Emitter voltage drop may be more important. Here's a graph of IC vs VBE from the Vishay BC337/338 datasheet. 



At 25 &deg;C and IC = 20 mA, VBE is ~0.65 V. Note the strong temperature dependence. This is why you need to use a diode to set the bias voltage, and it should be thermally coupled to the transistor so their voltages track.