As you know, I use MDF for most of my buildings. I’ve got enough of the plain sided kind to last awhile but I’ve been hearing that some folks can’t find it anywhere. Jon Davenport found this at Lowe’s and it looks like it would be an excellent substitute!
http://www.lowes.com/ProductDisplay?partNumber=355960-46086-152541&langId=-1&storeId=10151&productId=3402946&catalogId=10051&cmRelshp=req&rel=nofollow&cId=PDIO1
Get ye to a Lowe's, Alfrik! (Sorry for the continual misspelling, my friend. I'm such a dolt!!)