Some of our best pretend toys are wearable. We have received many costumes from older cousins. My daughter got a box full of princess dresses from her cousin in Maryland earlier this year -- what a thrill! My boys have inherited superhero costumes complete with stuffed abs and biceps. If you don't have older cousins, garage sales are a great place to find costumes, especially now that Halloween is past.
A few years ago, Santa brought my five year old (then three) a whole box full of hats. He was so excited to become a race car driver, a cowboy, a viking, a pirate, or a fire fighter just by changing his hat! If you think Santa might want to do the same for your preschooler, I would recommend buying new rather than used. One website with a great assortment of hats and costumes for kids is Daddy's Toy Shop. Mastermind Toys also has a good selection.
Of course, there is more to pretend than just the clothes. My daughter loves the play kitchen she got last Christmas. I paid half price when Target was clearing the shelves to get ready for after-Thanksgiving shoppers. Now is the time to look for big items if your local stores haven't already stocked the shelves with Christmas toys. Many of the big-ticket toys may be on clearance.

We also enjoy classics like the Fisher-Price Cash Register and the Fischer-Price medical kit. The pieces of these toys are easy for young hands to hold and are the perfect tools for children to set up their own store or hospital. Because these classics are indestructible, they are easy to find at garage sales or on ebay.
Last year, my children got the Medical kit for Christmas. It comes off the shelf any time any of us are sick. I think they like giving pretend shots the best. At the time of this writing, I've been vaccinated about 6 thousand times.
To me, the best thing about pretend toys is that they give parents a chance to experience the way their child views the world. It is easy to have fun with your kids when they are pretending to be just like Daddy or their favorite superhero (or both... SuperDad!)
Admit it. Don't you like pretending sometimes, too?
-Midwest (Super)Mom