Specialized Allez Sprint Fork I have a 2019 Specialized Allez Sprint Frame. I need a replacement fork, but specialized does not stock them. Are there any aftermarket forks that will fit this frame? If so, do they require a new headset?

Here is a photo of the OEM fork illustrating the integrated race and gap below the race, which I believe will make it challenging to find an aftermarket fork that will fit.
Aftermarket forks are available for pretty much any bike frame.

To fit the frame there are three things to consider, length, steerer diameter and taper, and brake mounting.

The length is usually measured 'axle to crown', which means the center of the axle to where the fork meets the frame head tube.

The steerer diameter and taper (larger size bottom bearing) have to match the headset bearings in the frame. I believe the 2019 Allez Sprint has a 1-1/8" to 1-1/2" taper steerer.

Obviously you need either a rim or disc brake fork.