The four of them had transported to an alternate universe.
There, they gained special powers as their physical body was altered in dreadful ways.
Their leader, Reed Richards was Mr Fantastic.
The remaining three are The Thing, Human Torch and Invisible Woman. 
Among the Fantastic Four costumes that one can presently buy in the market are Mr Fantastic Costumes and The Thing outfits.
The Avengers
The Avengers are a group of superheroes in the Marvel comics.
Its first appearance was in the September of 1963.
It first consisted of Iron man, Thor, Ant-Man, The Hulk and Wasp. Captain America later joined them on the 4th issue.
In the series, one of the main themes is that the group of superheroes come together to fight villains that no superhero can withstand alone.
This is why they have the battle cry ‘Avengers Assemble!’
For a long period, this group of superheroes has featured humans, aliens, supernatural beings, previous villains, mutants and even gods.
Writer-editor Stan Lee and artist Jack Kirby created the Avengers.
There is numerous Avengers outfit for anyone to buy today.
Some of them are Captain America costume, The Incredible Hulk outfit, Thor costumes and Iron Man outfits.
The X-Men
This is another group of superheroes by the Marvel comics.
It is yet again created by both Stan Lee and Jack Kirby.
It first appeared in its first issue in September 1963.
The X-Men are mutants that are tutored by a certain professor named Xavier.
He uses their powers for the benefit of mankind.
The professor had recruited the likes of Beast, Iceman, Jean Grey, Cyclops and Angel.
He had called them X-Men because they had special powers that came from their possession of the ‘X-Gene.’
This gene is not present in the body of the normal humans.
It is what gives the Mutants their gifts.
Because the humans don’t have it, they perceived the Mutants as a threat to their existence and often went to fighting with these Mutants.
Notwithstanding, in the event of impending dangers or threats against humanity, it is these X-Men that come to the rescue of humanity.

There are plenty X-Men costumes that are available, some of which are Emma Frost and Wolverine outfits.