TMI sells complete aftermarket seats that recline & have "semi" OEM (year specific) covers for chevelles. Example for 1969 Style:
1969 CHEVROLET CHEVELLE TMI Products 47-8019-3048 TMI Pro-Series Low Back Seats | Summit Racing
OPG/Classic Industries sells Procar Rallye aftermarket seats with oem style covers they call "Touring II" Example for 1970 style:
Distinctive Industries 090456IVORY: 1970 Chevelle / Elcamino Front Buckets Touring II Assembled Reclining Seats L-3895 - JEGS High Performance
Dynacorn Industries sells a full reproduction bucket seat 69-72 style... most catalogs show these complete seats, and the complete bare frames/springs- however be aware that these use inexpensive PUI covers and hard as a brick PUI buns, and overly stiff reproduction springs. I would not recommend going with them.
They also do not make complete 67/68 side release style seats to my knowledge, although you could probably order the 69-72 style with the earlier covers on them.
For the price of the full repops, you are better off finding a decent set of used originals, then having them redone with quality covers and buns.
I will say, for the price of the full aftermarket ones though... Firm bolsters, reclining... They are damn tempting. If I had a car that was not attempting to be 100% original, and didn't have bucket seat cores, i would be all over a set of those TMI or Procar seats with oem style covers. I've sat in both and they are really day & night improvement over even restored 50 year old seats... but then again, if you have to have original- nothing but that will do.