Is there (something)(clause)?

Here are some examples of "Is there ___?":

Is there a day when we could meet and discuss this in depth?

Is there anyone I can talk to about the status of my application?

Is there something you wanted to tell me?

Use this kind of sentence when you want to ask about whether a certain kind of thing exists.

Use "Is there ___?" in situations where you need to be polite. When you ask a simple question like:

What size are you looking for?

...the question assumes that the listener is looking for a size. But maybe he isn't. Maybe he's looking for a certain style instead of a certain size. So it's more polite to ask:

Is there a certain size you're looking for?

a certain (thing)

The word "a" can mean different things. It can mean something specific or something general. For example, the phrase "a movie" can be used specifically:

I saw a movie last night.

Or it can mean something general:

It's fun to go out to see a movie every once in a while.

But when you use the phrase "a certain ___", it's clear that you mean a specific thing.

I made the mistake of telling a certain friend about my problems with Jeff, and next thing you know the whole school was talking about it.

Let's just choose a certain day to meet for a few hours each week.

look for (something)

The phrase "look for" usually needs to have something after it:

I'm looking for a gift for my husband.

But in questions, that part comes earlier:

What are you looking for?

There's an old rule that some English teachers might try to enforce, which says that you shouldn't end a sentence with a preposition like "for". According to this rule, you're supposed to say or write:

For what are you looking?

But this is really strange and old-fashioned. You can use it in extremely formal writing, or maybe as a joke.