It depends on the game, but in general, no.
In Gen 2, which forms appear depend on which of the puzzles in the ruins have been solved, with all forms only being available once all four puzzles are solved. However, even once all puzzles are solved, the Z form has a lower chance of spawning due to the way Unown's form is determined in the code (Z has a 6/256 or 1/42.6 chance while all other forms each have a 10/256 or 1/25.6 chance). The punctuation forms (? and !) do not exist until Gen 3.
In Gen 3, Unown can only be found in the Tanoby Chambers in FRLG, with each chamber having different forms at different spawn chances.
DPPt, works much like Gen 3, with different Unown forms being available in different rooms of the Solaceon Ruins. 20 of the 28 forms appear in dead-end rooms at an equal 5% rate, each, while the 6 remaining letter forms each have a dedicated room at 100% each. The final two forms, ? and !, share a room at 50% each.
HGSS work similarly to Gen 2, with different forms being unlocked depending on how many puzzles you've solved. However, once all puzzles are solved, all of the 26 letter forms are weighted equally in the lower chamber. The final two punctuation forms appear in the upper chamber at 50% each.
Unown cannot be caught in Gen 5 or XY.
Unown of every form can be found in a mirage cave in ORAS, south of Route 107. This is the only location in the franchise where all 28 forms can be found at an equal rate.
Source 1, Source 2, Source 3
