Having high quality nude reference images is essential. Seriously. It's essential for understanding the human form and accurately representing that form with or without clothes, armor, etc. If you live in the country like I do, finding a live model isn't so easy. Up until recently I had a bookmark folder of my favorite online nude model resources. I accidentally deleted them! Thankfully, all hope was not lost. I asked some of my artist friends for their favorite sites and in process rediscovered all of my favorites too. Below are a handful that I use the most.
This site isn't free, but it's only $5.99 per pose and each pose consists of around 24 high resolution images of the pose from every angle.
DeviantART can be an amazing resource for reference images. But you must check the users rules for using their stock. Some are more strict than others. As for nude stock, there's just about everything.... And I mean everything. Some things you might wish you'd never seen. But mjranum-stock is my go to if I've not found what I'm looking for on Posespace.com.
The two sites below are excellent for timed sketching. Practice is something every artist should continue to do, no matter their level of skill. It's how we keep growing!
Hope you find these useful!