Search for a millwork shop. They should be able to build you one. Many lumber yards have millwork shops too, so you can try them.

If you are replacing the frame, just go with either a new construction window or a full framed unit.

If you are deadset on replacing it with an insert, most window frames are just comprised of 3/4" thick material in a depth that is relative to the wall depth.

Nate, we replaced our old sashes with sash kits rather than going the pocket window route.

(Had several contractors look at the situ and they all pushed pocket windows; when sash kits were mentioned, it appeared that none of them knew what sash kits are or that they even exist!). Ended up being a DIY project.

Quite easy to install in our case. Just removed the old jamb liners; replaced with new jamb liners from the sash kit and popped in the sashes. (Maybe also the parting stops were removed but can't remember for sure.). Existing interior and exterior trim is left intact and all the work is done on the interior side. No ladder work on second floor windows.

The other advantage over pocket windows is that you don't lose any glazing area to pocket window frame. This was important to us because we have a lot of shade and froggy gray days.

We used WeatherShield sash kits. Purchased directly from the factory during a Customer Appreciation sale. (Factory is within 100 miles of where we live; placed the order and windows were ready for pick up within about 3 weeks, IIRC.)

We got unfinished pine so there was extra work in finishing. Didn't get clad because the windows are all under a two story porch so not exposed to the elements. Huge improvement over the old single pane sashes.

https://www.weathershield.com/Category/Sash-Kits

If you have the frame From the window being tore out just copy that. like Eric said, most are just 3/4 sides and top but would also pribaly have a 1 /2 thick sill. If you wanna proceed let me know and. I can try to walk you through it.