try the medela website or contact the lactation dept of your local hospital. the website should have a retail locator too.
I have the same problem with my tubes this go around. I never once had to replace my tubes while I EPed for my son for over a year!!! But I recently purchased some tubes off of Amazon and they are great and more than half the price of the tubes straight from Medela. http://www.amazon.com/gp/product/B0033B9CZ2
I had a problem with my first set to!!!!!
FYI, if you have "stuff" (probably mold of some sort) growing in your tubing; it is possible that you could have mold growing inside the pump housing of your PIS.