The best thing about discovering bands that have been around for a while is that there's MORE out there.
For an impoverished soul such as myself, the
best thing is you can buy the CDs second-hand for cheap.
Most of the time, anyway. Some of the more obscure albums were never put onto CD, or are only available as an expensive import.
And don't forget the one-hit-wonders (e.g. Focus, Golden Earring, Kansas, Argent, Uriah Heep) that everyone's heard of but whose most interesting material never saw the Top 40.
Although maybe not "prog", I have to also plug one of the best concept albums of the 1970's (I consider it in the same league with Dark Side of the Moon): "Hope" by Klaatu.