You can say "Shichaku Shitsu wa Doko desuka?(Where is a fitting room?)"
Many of Shops accept credit cards.
You can try to find the card's stickers around cash counters or just ask staff " Card tsukae masuka?(Can I use a credit card?)"
You can also sell your clothes in some shops.
Because the shops I introduce in this website buy in clothes from end users. It means that you can also sell your clothes to the shops. But some of them don't accept imported items and average purchase price of the shop side is very low. So don't be surprised if you can't get enough money by selling your clothes. Of course you can deny selling if you aren't satisfied with the price.
You can say "Kaitori Onegai shimasu(I would like to sell my clothes)"
