You can get Aspen at the big box pet stores like Petsmart and Petco. I used to use them for my snakes, but yes they are expensive. I want to say I also found some at a Walmart once but don't quote me on it. It is recommended not to put snakes on pine but one time their habitats needed cleaned and I didn't have Aspen so I tried a few on Pine and have never had any issues so now they are all on pine. They're 13 years old now and this breed only lives an average of 15 years so I figure if they've done fine on Pine the last 4-5 years, they'll probably live the rest of their lives on it now.
I also use pine for my coop and brooder and have never had any issues. I prefer the really fine shavings over the big chips. The ones I get are called Swift Pick shavings and the only place I've found them is Ace Hardware where they are sold as horse stall bedding. I like the small size as it breaks down faster and is easier to clean out. My chicks start out on a cloth towel for about the first three days until they know how to find food in the feeder. Then I take out the towel and replace it with the swift pick shavings and from then on, that's what they're on since that's also what I use in my main coop.