Casey, depending what you are looking for, they can be fairly easy to find, or relatively difficult.
A flow control, can be purchased new, and the are not expensive. The problem with that is it doesn't come with metal tubes,, which means you end up using hoses. Not a big deal in my mind, and can be done to look quite nice. (I don't remember seeing a complete one of these come up on ebay,, with tubes, mind you, I've only been watching since 2014.)
Holding valves,, typically will cost around $200 US, and if you are super lucky,, it might have the tubes from the valve to the TCV. (holding valve with tubes to fit short frame 446's) Most times, you're not going to be that lucky. You can buy new tubes for these, but I'll throw a caution to you about that. I did purchase a holding valve for one of mine,, and I also purchased said new tubes that were supposed to fit. Well, those tubes never did fit, as they were off by about an inch or so. So, I did end up with hoses instead anyways. (I still have them, for what they are worth)
For one of my 446's, a 75 model, I did purchase the later travel control valve, with the built in holding valve on it. It's on the forum here someplace, on how I did it, and without drilling new holes in the floorboard either. The install went fairly easy, and as far as tubes, I reused the original tubes that came off the tractor, with slight re-bending of the tubes using heat. The integrated TCV, is a bit more expensive, then just a holding valve.
This is one example of a flow control valve,
https://www.princessauto.com/en/det...mpensated-adjustable-flow-control/A-p8252777e