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The alternator on my NB8A is starting to make some noises, so I'm guessing the bearings are getting a bit tired and I'm looking for some suggestions on replacement or fixing.

Are these alternators serviceable and are parts available?

Are the cheap eBay alternators OK, or pieces of junk?

Are there alternators off any other models that I'm likely to find at a pick-a-part wrecker that are a direct fit?

Does anyone have a decent alternator they want to sell?

If the brushes have not worn into the slip rings these are an easy Alt to split and replace bearings on.
It is often simpler to remove the front pulley bolt with the Alt still on the car with the belt in place if you don't have access to a rattle gun. Alternately pull it off and take it to your local workshop and get them to loosen this nut for you.

Remove the front Pulley nut and washer and wiggle the pulley off, be mindful of the shims and spacers behind it and the order they come off the shaft so they can go back on the same way.
Once the front pulley is off you only need an 8mm, 10mm socket and a Phillips head screw driver. Remove the 12V mount on the back, the long bolts that hold the case together and gently try to separate the alloy housings from the Iron Stator ring. The front will come off very easily.
For the rear you may have to turn the rotor while pulling on it to pop it out of the brushes.

To ease assembly I have found it sometimes helps to bevel the edges of the brushes to help the rotor slide back home. If they look excessively worn or don't have much travel left once the rotor is back in place look at replacing them. Any Auto elec should be able to solder in a new set, if they don't try to sell you a new brush assembly.

With the rotor out you can now remove the 4 Phillips head screws on the retainer plates that hold the front and rear bearings in their housings. Take them down to your local bearing and seal shop and get the guys to measure them up and cross match you some replacements.

Throw it all back the way it came apart, spin it up by hand to make sure nothing is binding and reinstall.

Last time I overhauled one bearings were about $16ea.
The Alt on my NA6 however had such worn rotor slip rings when the bearings got noisy that I could not separate it from the housing without destroying the regulator/brush assembly and got it fully overhauled including having the slip rings machined for $115.

Caution, tread carefully here, very carefully ...

NB8A alternators are regulated by the ECU, not by an on-board (or external) voltage regulator.

As a result, you need to fit the correct alternator, or alternatively fit another alternator that is either internally or externally regulated, with the appropriate wiring/voltage regulator. As far as I know, there are no ill effects for bypassing the computer for voltage regulation, but please do not take my word for it, do some checking with people who know (perhaps starting with a forum sponsor near you). Perhaps somebody who has done this might chime in here.