Discuss and learn French: French vocabulary, French grammar, French culture etc.
No, there's no standard spelling "y'a".
As you may now, in fluent speech, "il y a" is often pronounced effectively as though it was written "y a". But at that point, you're departing from standard spelling, and it's arguable whether you write "y'a", "ya" or "y a" or something else that might occur to you.
In internet chat (and presumably phone texting) it's ya.