Where are all the earlier games available?
As Qawsed Asap suggested, finding Daggerfall, BattleSpire or Redguard is problematic. They're no longer being manufactured, so all copies out there are it. You can sometimes find them available on eBay or Amazon, and there are legends of people finding them in bargain bins in retail brick & mortar stores occasionally, but don't bet on it. Arena is free for download from gamesas.
Also, what are they like?
Arena & DF are less "glossy" than Morrowind & forward, but they are FAR larger and much more wide-open in terms of gameplay. They are also quite a bit less forgiving. It's difficult to bork a character in Morrowind, but you can make ill-advised choices in TES: 1 or 2 and leave yourself up a creek and paddle-less.. They are worth the effort, though. (Well, I never played Battlespire.. so far.. I have it, but by the time I acquired it, using the VESA-based video was difficult; Now Redguard was a riot.. Straightforward, but very fun! Now, with the UESP Wiki instructions on using DOSBox, I plan to get back into both after I tire of Daggerfall, which might take a while)