I have bought three uppers in the last three weeks or so from their site. If you see one of the models you like, type in your email to be sent an alert when it is in stock. They are releasing small batches of a few models every day at random times for an hour or two before they sell out.
I believe they are doing this to stop people from buying up all their uppers and reselling them. Also, you could try randomly visiting the site and selecting the "in stock" feature when searching their uppers, but the three I purchased were all from being sent an email alert. There have been other times where I followed the email link a few hours later and everything was already sold out, so act quickly!