Both publishers offer their books one month before the official release day. Eg. If my book is releasing on April 1st, you can buy it from Harlequin on March 1st.
Other publishers have different schedules and arrangements, but if you are a reviewer, watch for my books on Netgalley. If I’m told my book is there, I will usually tweet it or add an announcement here on site. To be notified auto-magically, join my What’s New feed from my subscribe page.