The phrase "a supply of extra" is correct and usable in written English.
It can be used when referring to an additional quantity of something that is available for use or distribution.
Example: "We have a supply of extra materials in case we need to complete the project ahead of schedule."
Alternatives: "an abundance of extra" or "a reserve of extra".
Best Buy provided a discount towards any Guitar Hero World Tour-related product with pre-orders of the game, and provided a limited supply of extra drum bass pedals for the "Expert+" mode with purchase of the game.
Traditionally, video game protagonists are presented as Teflon-coated superheroes, emboldened by an endless supply of extra lives and, in many cases, ammunition.
The film was shot mostly in Haiti, where Jacopetti and Prosperi were treated as honored guests of dictator "Papa Doc Duvalierr, who afforded them diplomatic cars, the right to film anywhere on the island, a nightly chow down at the palace, and a limitless supply of extras – the often naked black bodies who pepper the film's landscape with disturbing density.
In the five years before the latest crisis, I used up a lot of my blessed supply of extra time by reading.
The XFL also involves the dream that assures an endless supply of extras for however long the xtroverted show lasts -- the dream of being discovered by N.F.L. scouts, of getting an opportunity to go to the N.F.L. and play real pro football.
It is a strong inhibitor of keratinocyte proliferation [34], [46], which interferes with the supply of extra keratinocytes to cover the wound bed.
And computing companies have found plenty to do with the continual supply of extra transistors.
There are also questions on Wall Street about whether the relatively strong car sales this year will last once the supply of extra buyers attracted by lower prices has been exhausted.
These early farmers were clearly successful, though life would still have been precarious, with hunting providing precious supplies of extra protein.
