Some websites are better than others at sending out notifications. I haven't received any from Palmetto in over a year, I think they stopped. I had one set up at Brownells and actually found the thing I was signed up for in stock one day and never received an email. I had a notification set up for a lower and BCG at Ballistic Advantage and they had some inventory a few weeks ago that I never got an email, thankfully I checked and got what I needed. AIM Surplus, Arm or Ally, Bravo Company, Aero Precision, Midway have all sent me emails recently.
It's a crapshoot right now. I recommend checking daily for the parts you want in addition to having email alerts. What are you specifically looking for? Someone may be able to help with some obscure retailers that don't generally show up in a google search.