ruger #1.
Ruger used to sell them this way but when they went to POS(point of sale) they let them run out.
Dakota still sells barreled actions but they are not cheap, worth it but not cheap.
Most major makers will sell there actions if they have them to sell, when there made overseas it get tough but there are some great CZ 550 actions, not single shot but built for .416.
There was a company making single shot falling block actions, for the life of me i can't remember there name but i will look around here, i have one of there catalogs.
The #1 is good to build on, there are some aftermarket parts and custom work for these to make them a side lever, works better when bench rest shooting.