Maybe a trip to a costume shop, lingerie shop, toy shop or even a thrift shop might be a fun trip for you both. But, until then, things don't have to be elaborate. For a maiden outfit (though, it isn't going to and doesn't have to be exact period) you could get a long white shirt or nightgown and a vest, along with a skirt. Men are fairly easy. If you get a good set of boots, tuck your pants into them, and a gain a simple white dress shirt will suffice in making a fairly decent Master outfit.

A little white dress and some mary janes could make a "good girl" outfit. And, as mentioned, a regular plain old suit can make a "daddy" or a "boss" or a very sexy and sleek refined Master on his lunchbreak. lol

White shoes and white dress, with a little paper hat and a little red cross painted on it and instant "nurse". And, well, a doctor all he needs is a white coat. A thermometer, a popsicle stick for a tongue depresser.

How about a good pirate kidnap scene? Again, the boots tucked in the pants, an oversized shirt, and a scarf. If she has an old prom dress and dress shoes she can be a princess, just make her crown or tiara out of a hairband and some glitter.

The most important thing you need for roleplaying isn't even clothes. It's your imagination. Granted, clothes and props are nice but when it isn't possible then your good old imagination should be able to do the job. When I was a kid, I didn't need much to become a princess or a slave. It's more in the attitude than what you are wearing.