Edit: There are still some $797 fares available, but there’s much less availability now.
JetBlue has some excellent fares in Mint available between Boston and San Francisco. These are pricing out at just $399 OW / $797 RT for nonstop transcon travel in lie-flat beds.
You can find these flights directly on JetBlue, Google Flights, etc., so they should be bookable anywhere.
Secret Flying has a long list of dates where you can book this deal, with availability from February all the way through to the end of October.
For example, here’s an option for round trip from May 7 to May 16:
Although the flight from Boston to San Francisco is a red-eye, the lie-flat beds will help mitigate that.
As mentioned above, there’s a ton of availability over the course of the next eight months. However, this deal could end any time and some dates only have a couple of seats left at this fare, so it’d be best to jump on the deal if you can make use of it.