The retail owner of the shop I use is a great lady that has a lovely community spirit and just wants to help us out - she doesn't charge us for the use of her shop.
We are also looking at using a gallery space for a couple of weeks to set up as a shop. I've heard of some people doing a short term lease on an empty shop for a few weeks or months, which also benefits the building owner while they are between leases.
Here is a link of what some companies have done (some look like the cost a fair bit of money. But there are cheaper ways of doing it)
http://www.businessinsider.com/17-most-creative-pop-up-storesI recommend doing a pop up shop in collaboration with other people because you all bring in your own customer base and have more advertising opportunities together. Also for me I would find it hard to fill a shop.
I would love to hear if either of you do pop up shops and how it goes for you... good luck!