Hmmm, well, clothes shopping is not really my forte, but I'll try to be of help.
I'm going to assume that the ones shopping are the captors for her, and that they are probaly going to get her clothing that she will not like...
I would suggest, that they go to some outlandish clothing store, and either (1) The guys make her tell them her clothing size and then one of them goes inside while the other watches her then buys the type of clothes he wants for her. Or (2), they threaten her to keep quiet and act as if everything is normal and watch her like a hawk in the store, and they all buy the clothing.
Number one would probaly be more logical, since she would draw attetion to herself in the store b/c as of right now, she does have a lack of clothes.
That's all the options I can think of, but if you wanted help with HOW they are to shop for clothes (like in the store, looking at clothes, ect), then just message and once again I'll try to help.
Hope I was of some use, and can't wait for the next chapter!