The easiest way is to buy some but the other easy way for feminized seeds is to continue to flower your plants after they are ripe. It's a procedure called rhodelization and it was developed by Soma. About 2 to 3 weeks after the trichs are amber the female will attempt a last ditch effort to carry on the genetics and will throw out some bananas. Colect this pollen. It is female pollen and when you use this pollen on a younger female that is about mid way through flower you will end up with feminized seeds. It's not a fast procedure but has been proven to work.