I apologise in advance - I haven't been on this website in a while and have no clue where anything is. Woo-hoo. Thanks!
If you're looking specifically where to buy backdrops, the best places would probably just be from the shops of other users! Backdrops are wearables and they are pretty popular so they can come from collections, vending machines, specialty event stores...pretty much anywhere you can imagine, depending on the item. Alternatively, if you are just looking to see what each backdrop looks like, go onto your wardrobe page, click, "Show all items" and then select the background option and it will show you all of the backdrop items on subeta. Sites like Arvee link or http://www.subetalodge.org/ can also be helpful.
Hope that helps!
You're welcome. Hopefully that helps. (Also, you should go pick out your tree, it looks like you haven't done that yet!)
Not to mention that you can also preview all backdrops by using your wardrobe. Go to the clothing creator find the drop-down menu that says "Use Clothes From:" and select 'All Clothing on Subeta.' Then click 'Backdrop' from all the available layer selections. This will show you all the backdrops currently available on the site. The preview window on the lower left side will show you the name of any backdrop you select.