Well, one good source is gunboards.com they have a dedicated 98k forum. A great reference to pick up would be "Backbone of the Wehrmacht" by Richard Law. While not perfect, and hotly debated by hardcore collectors, it is a good basis on which to begin. He does cover all year/make code combos, history, correct part markings, etc.
As far as the Mitchell's...well, I have my own opinions on them. Personally I think they charge WAY too much for humped up, post war, no history rifles. They are misleading in some of their ads which would lead some novices to believe they are getting mint German 98k's when they are really getting post war Yugo's.
If you want to find a 98k, you can easily go with the nyriad of "Russian Capture" pieces all over the milsurp market for $350 or less. They are Frankensteins, since the Russians took all the captured 98k's, stripped them down to component parts, refinshed them, and put them back together with no concern of matching numbers. But they are genuine German 98k's that saw service on the Russian front....
You can still find original, non "RC" 98k's too, from $250 up to $1200 depending on condition and originality. Granted many of these are comanding higher prices nowadays, good bargains can still be found.
If you really want an original German WWII 98k, don't settle for an M48...but be careful, they are like potato chips, you won't stop at one....I now have 18, no RC's amongst them, most are bringbacks....