How often does a headset and/or bearings need replacing? I've had my road bike a while but never really bothered to replace the headset and/or bearings.

Looking at new ones online, I have no idea what parts I'd need, seems there's a ton of slightly different headsets.

Mine is a "AHEADSET" VP-A14A, but I can't seem to find that part.

They last until the races are shot....which considering that headsets are mostly static....means they last pretty much forever unless they get filled with water and rust. You hit a hell of a pothole and manage to not break your bike, you might index the races against the balls but that is the only real other way you'll trash a headset.

"If it ain't broke, don't fix it".
Headset bearings have it fairly easy, only turning a couple of degrees at a time. They can't cause anything nasty by seizing suddenly. Particularly for a road bike, about the only damage that can happen to them is that they develop a degree of notchiness over time. If yours hasn't done that, i.e. bar still turn smoothly, there is really no benefit to replacing it.

That have cartridge bearings or loose balls ? Ahead (type "threadless' or trademark Cane Creek) set headsets
come in both types depending on your Price.

where do you look? this low cost OEM parts ?

or online The Manufacturer.. Headsets

The bearings look like this https://www.hashtagbikes.com/v/vspfil...ges/HD0022.jpg

In all honesty, a good cleanup and re-grease seems the best option here, there's no excessive wear/damage.

That is Loose ball in a retainer cage .. you can find new caged sets or press them out and replace all the balls. ,
[grade 25 better, come in bags of 1000.**]
In most loose ball headsets ( less convenient, for fast assembly or packaging, so factories don't do it)

the ball will stick in Bearing Grease without needing the retainer . Threadless or Not.

** I buy them from Industrial Bearing suppliers ,[In Eugene it was convenient to just Go to the Shop]

The only maintenance needed / possible is periodic re-grease and occasional ball replacement.

If you take those caged balls into an LBS they will sell you exactly the loose balls you need. It will cost more than just ordering the balls online, but that's fair for the knowledge. What you'll then do is: flip the bike over, fill the lower bearing cup with grease, insert enough balls to completely fill it, remove one, insert fork, holding fork in place to keep the balls from escaping, flip the bike back over, grease, balls, reassemble.

The main failure mode of headsets is the bearing cups get dimpled by the balls this makes them feel "notchy." Going from caged to loose balls changes the contact points and will reduce the problem.