"if there's a need" is correct and usable in written English.
You can use it when you are expressing a possibility or condition. Example sentence: I'll bring extra supplies if there's a need.
"What Y's have always done is, if there's a need, they get creative," said Laura Slane, associate director of program development/aquatics for Y.M.C.A. of the USA.
(Still have plenty left, if there's a need).
I have no problem stepping up if there's a need for a doctor onboard a flight.
You can always resume the therapy when and if there's a need.
"She's nervous," Alicia says, as if there's a need to explain.
If there's a need for me to be around longer than 9-5 unexpectheny, therethere's that flexibility.
"Instead of wasting public money, if there is a need let's come to a sensible solution".
The accused's lawyer is there to object if there is a need".
If there is a need to meet, I am ready.
If there is a need we will wait all night".
"If there was a need for the theater 50 years ago, there is a bigger need today".
