I just checked my pet chicken and they're sold out of female silkies. GCAs GC_Raptor mentions, MPC does offer sexed silkies. It's an additional $12 per chick, for a total of $16 each.
If you do go through McMurrary I'd order at least 6 to make sure you get 2 pullets.
... so I can blow it all on more chickens 
whoa! Is there a reason for that number for silkies?