Some packaging suppliers online sell it, such as uline, freight might be pricey. You may have a local source, would probably take some searching around. I work for a packaging company currently so I haven't had to source it. No, nothing wrong with printed newspaper, I just prefer unprinted dunnage because it won't leave stains on albino snakes.

As far as preference, I can't really say. I've kept the same snakes on either at different times and haven't noticed anything. I would assume they prefer aspen because most snakes will burrow from time to time.
I use the paper because of the number of reptiles I have it makes for a much easier clean-up and less hassle. I like being able to quickly clean an entire stack of 4x2 cages.