Example Fire, Fira, Firaga, Firaja, ??? Has any source ever mention higher tier than -ja spells?
Has any source ever mention higher tier than -ja spells
sorta. while Frank's comment sounds correct (not sure where i heard it but seems correct to me too) there is (as of 27/02/2018) actually no Thundaza, Firaza, Blizzaza or Waterza as all these pages on the wiki don't have pages to them, but if you replace the z with a j in their URLs you'll get the -ja level spells.
this is also backed up on the wikia's template for Black Magic Template where under reoccurring we only see up to the -ja level and infact -ja is the highest level (so it's not a matter of Frank just getting the suffix wrong, there's no magic after -ja)
But ofcause this is just for named spells following this pattern, if we consider -ja level spells as being Level 4 (Normal, -ra, -ga, -ja) then we do have Level 5 spells
in Crisis Core: Final Fantasy VII Elemental Magic Materia is actually split between levels (unlike what it was in Final Fantasy VII where the Lighting Materia would grant Bolt 1-3).
in Crisis Core you have the inital 3, Normal, -ra and -ga but after this the next most powerful Magic Material after -ga is Dark [Normal] (ie. Dark Thunder). these, just like their regular one, are split between Normal, -ra and -ga so Level 5 Magic where would be Dark -ra spells.
it would also be noted that after Dark there is Hell -ga spells where they skip Normal and -ra so you go from Dark Thundaga to Hell Thundaga
Source: Magic Materia (Crisis Core)
Also in Kingdom Hearts it was slightly inconsistent. while they did feature -za this was infact the 4th level and was sometimes also -gun but in later installements to the series -ja was used for the 4th level
The Kingdom Hearts series utilizes the tier system used from the Final Fantasy series to denote its levels of power with the same suffixes -ra and -ga for its spells. The exception is the fourth level, which tends to vary across the series, typically using -gun or -za, but later using -ja in later installments.