My go-to trick, especially for architectural backgrounds, is to use the crap out of Google Sketchup. It's a free application you can download, and there are a TON of free 3D models in the 3D Warehouse that you can look up, all of which are free use models so you don't have to worry about copyright issues.
https://3dwarehouse.sketchup.com/?hl=en
If if you're looking for, like, a cathedral, you search it in the 3D Warehouse, download it and open it in Google Sketchup, and turn it in the program to the angle you need. Then screencap it, drop it into whichever program you're using to draw, and use that as your template for drawing your backgrounds.
Also, even if you use heavy reference like that, you still need to have a basic understanding of perspective in order to make sure your characters are situated with the backgrounds properly and that your backgrounds make sense. I personally really like "Perspective for Comic Book Artists" by David Chelsea. It's all drawn as a comic with his two characters talking about and interacting with the perspective around them. It starts out pretty basic with the easy stuff you need to know for most situations, then gets really weird and deep with NASA-levels of math at the end. It's highly entertaining.