I've looked at dozens of websites, more than half associated with universities, and I still can't figure out which foods are high in purine and which are not.
What I do is try to compare the purine content, like per 100 grams, and go from there. But many guidelines don't give you the number, and instead say which ones are "high" and which are "medium" or which are "safe" to eat and which not.
For instance mushroom has been mentioned as both high purine and low purine. One site mentions tuna as something to be avoided while another says it's safe to eat. Some sites mention the actual purine content but only for a limited number of foods. And I'm not sure how accurate their numbers are, given that other sites don't agree.
For instance, Wiki links to this non-university page which gives purine content for a limited number of foods: dietaryfiberfood.com: Gout: List of High and Low Purine Foods to Pay Attention
In any event, I'd appreciate it if you could link me to a paper or book or site that is both comprehensive and accurate, in terms of listing purine content of different kinds of food expressed in measured numbers.