score:5
What your friend told you it's pretty inaccurate and the opposite is actually true: many people at the stadium DO wear jersey / shirt of their favorite team.
Authentic jerseys are sold pretty much everywhere at the same price, which is usually quite high. You can find "accurate enough" jerseys for lower prices, tho. Usually outside the stadium there are a bunch of sellers but prices are somewhat higher than those you can find in local markets during a non-match day.