Tシャツありますかって英語でなんて言うの？

You may ask directly an assistant in the shop:
"Excuse me, do you have any souvenir tee shirts?"
Or, you can express what you are looking for:
"Hi, I'm looking for a souvenir tee shirt."
Of course, an assistant would respond to this by either stating that they have some tee shirts which you may be interested in - or by mentioning that they don't have any tee shirts.

A simple way to ask this question is to say, "do you sell T-shirts?" It would be polite to say "excuse me" beforehand.
"Where are your T-shirts?" - where in the store are the T-shirts located
"Do you have T-shirts?" - the same as asking if they sell T-shirts

A: Excuse me. Where are your T-shirts?
B: They are in the back on the right hand side.

At a souvenir shop, you can ask whether they have any T-shirts in a number of ways.You can use any
of the above statements. Here are some other examples:-
1. I'd like to give someone a T-shirt as a souvenir, do you sell any T-shirts?
2. I was hoping that you sell T-shirts, I'd like to buy one.
Hope this helps :)

"Do you sell T-shirts?"
This is asking if they have any T-shirts, that you can buy.

"Do you stock any T-shirts?"
"Stock" means you are asking if they have any T-shirts in the shop, maybe not where you can see them.

If you are wanting to now if someone or a shop has something then you would ask 'do you have?' or 'do you sell?' by adding t-shirts you are explaining what you are looking for

>Excuse me, do you have a T-shirt section?
>Hi, do you perhaps sell t-shirts?
>Do you by any chance sell t-shirts?
..........................
Example 1
Him:Hey can I help you?
You:Hi, do you have a T-shirt section?
.................................
Example 2
Him:Hey can I help you?
You:Hi, do you perhaps sell t-shirts?
...............................
Example 3
Him:Hey can I help you?
You:Do you by any chance sell t-shirts?
.................*.............

To ask a souvenir shop if they have t-shirts you can ask "Do you sell t-shirts?"

When you walk into a souvenir shop and want to find out whether they have t-shirts, then you can say:
-Do you have t-shirts?
-I'd like to know if you have t-shirts
-I am looking for a t-shirt to buy as a souvenir

If you want to ask the staff at a souvenir shop if they have T-shirts, you can go up to an employee and say:
"Do you have T-shirts?"
"Do you sell T-shirts here?"

If you want to know if the souvenir shop sells t-shirts then the best way to do this would be to say "Do you sell t-shirts?".

To ask if there are any T-shirts being sold in the store, use the sentences above.
You might want to buy a T-shirt for yourself or for someone you care for as a souvenir to remember a certain holiday or destination visited.

On sale: being sold at that time.
In stock: available

"Hi, I am looking for a T-shirt to keep as a souvenir, do you have any in stock?"