Packet Archive: This is one of the key resources --- a website with thousands of quizbowl packets of all kinds stretching back years. It’s important to keep tabs on their difficulty --- it contains things all the way from American middle-school questions (fairly quick and easy) up to extremely difficult open tournaments with horrendously obscure answerlines, as well as a number of specialist subject packets. If you would like suggestions for academic tournaments to start out with, try British Novice, ACF Fall, or ACF Winter for something more difficult.
One can also find so-called “trash” packets on the archive --- these are packets with a non-academic focus, including pop culture, sport and memes. Many are one-off packets written for fun, but there have been some tournaments in this vein, such as ACRONYM or the 2016 Lowbrow Oxford Open.
QBreader: This is a website that you can use for practising quizbowl questions on your own. It also serves as a database, allowing you to look up questions by category and tournament difficulty. This comes in especially useful when writing questions, as a sanity check for difficulty.
NAQT You Gotta Know: NAQT is an American company which produces a lot of quizbowl questions. One of the best features on their website are these short guides which go through and explain lists of topics which come up frequently in quizbowl tournaments.