Any standard 12v alternator will work fine at 48v.
As rpm increases voltage increases.
In most vehicles the alternator is driven about x3 or x4 engine speed. The cut in voltage where the alternator light goes out might usually be about 500 to 700 engine rpm. That is often around 2,000 alternator rpm.
To reach 48 volts the alternator will need to be doing about 8,000 rpm, and a bit more than that to supply full rated current.
That sounds horribly fast, but if your vehicle engine is regularly taken up to 5,000 rpm, the alternator will be x3 to x4 that. They are quite happy to run at 20,000 rpm continuously for year after year so 8,000 rpm is nothing.
You will obviously need to remove the inbuilt voltage regulator, and home brew something for 48v, but that should not be too difficult.
The diodes will be fine too, most are rated at 200v or more, and you will have zero diode problems with the original diodes at 48v.
Only real down side of vehicle alternators is that they are not very efficient. They are made to be very small with not much copper and iron inside. If your vehicle has a 200 Hp engine it does not really matter if the alternator takes 2Hp or 6Hp to charge the battery, so mechanical efficiency was never a strong design goal.
Small size, reliable, and cheap, yes. Efficient no.