Look at Bed, Bath and Beyond, Walmart, Target, Penney's, Sears, Macy's, Nordstrom's, eBay, Amazon, etc. I like going to the store itself for things like this so you can get a better idea of the actual color and feel. Make sure you get the correct size because some dorm beds are different.

Go online and you will find numerous online linen stores. Explore them and find the best piece for your home online sitting at home.

Comforter like a quilt/bedspread, or comforter like a friend with benefits?

Look on different designer websites.

I am moving into my apartment styled dorm in August. I have my own room with a full sized bed. I like the colors blue, green, and sometimes pink. I want a comforter. I want it to be along the lines of preppy. Solid preferably only because I feel like it would match easily with sheets, etc. Thank you so much in advance :))