I doubt you'll find obscure or a wide range of Sci-Fi/Fantasy books in Wal-mart on the physical shelves, but you buy books from their website and I'm willing to bet you can find more palatable authors on their shelves. For example, you might find Brandon Sanderson but I doubt you'd find R. Scott Bakker or Steven Erikson. One of the primary reasons for this would be how the story is told. Wal-mart is fairly known for their stance on appearing 'family friendly'. I'm willing to bet they don't buy many books that are explicit or violent etc. Grim dark has been quite prevalent and even books that are less grim dark are still likely too violent for Wal-mart to sell in stores. That being said, you can buy them from Wal-Mart.com and have them shipped to the store. Example: Despite the fact that you'd never see a Steven Erikson book in Wal-mart, you can buy the series on walmart.com.
You're more likely to find YA Fantasy on Wal-mart's physical shelves because it speaks to their family friendly branding.