stand out the front of the maternity hossy and catch them as they go into labour lol.

i got mine from our twin club- she advertised in our local twin newsletter (she is also a twin mumma)
also try your local MCHN centre- put a notice up on their board for second time mums!!
also local playgroups, ABA etc
Perhaps even contacting a well known, experienced doula, to see if you can tag along to her births to watch and learn and get experience that way!!