Correct spelling, explanation: sponsor is a noun or a verb that comes from Latin derived from spondere, conveying the meaning of promising. Although in English suffix er is used to make the nouns from the verbs, in this case, nor er neither or is a derivational morpheme, changing the grammatical category of the word. This word is simply a root and English learners need to remember its correct spelling as sponsor, not sponser.

Definition of sponsor:
1. noun, someone who gives money participating in charity events; a person that makes himself responsible for someone, especially with a religious aspect, for example, a godfather,
Susan got an interesting book from her sponsor father.
It’s commendable that they became sponsors for disabled children.
2. verb, give money to someone who needs help,
They usually sponsor poor children from Ukraine.
This company will sponsor scientific research.

Phrases with sponsor:
co-sponsor – a person who gives money to someone with another person or company,
They were co-sponsors of my mother who needed cancer treatment.

Incorrect spelling, explanation: both in American and British English sponsor can be pronounced as /ˈspɒn.sər/ where the sound similar to the vowel e is heard. This is the reason why some English users misspell the word sponsor, writing sponser. However, sponsor stems from the Latin word, and therefore, this is the only correct form.