I have a few older electronic ballasts, the oldest being
this Advance from 1999. Oddly I believe it uses a fully electronic design. It does have what I call a full sized case, it's actually the exact same size as a standard magnetic ballast. I believe it was made shortly after Advance dropped the Mark V branding for fully electronic ballasts.
My second oldest is
this Sylvania ballast. It's actually a relabeled Motorola ballast, Motorola used to make ballasts until the 2000s when they sold off their ballast division to Sylvania. Like the Advance from above, it's also a fully electronic design. Interestingly, it's rpaid start electronic ballast which is quite rare to see. The removable leads was also a thing that Motorola liked to do with their ballasts.
It's not a T8 ballast but I also have this
Advance F96T12 from the early 2000s. This one is quite interesting, it's a electronic ballast with high frequency operation but some of the elements inside are magnetic and it even has the hum to boot. I have a T8 one from the same era that's probably built the same internally. It also has the same characteristics.
Finally, during the early days of T8, it was common to use magnetic rapid start ballasts for them.
Here's a example I have from 1986, fairly early for T8 which were only introduced 4 years prior. Magnetic T8 ballasts continued in production until sometime in the 2010s but most places started using electronic by the late 90s. The late 90s seemed to be when you could see both types being installed. My high school had an addition built in 1999 and it was originally equipped with all magnetic T8 ballasts.
There was also the Advance Mark IV PowrKut ballasts, those were actually just standard magnetic ballasts that had a extra electronic circuit in them to reduce the cathode heating supply after startup to save a few watts. The ballast was still for the most part a magnetic ballast with regular line frequency operation.
If you're hunting for a older electronic ballast, you could try searching the part numbers in the pictures or just keep your eyes out for any electronic ballast that's a major brand and is: 120v only and has a full sized case. I think after 2005 or so, multi volt electronic ballasts pretty much got rid of the 120v only models.