You can find the T-shirts and hoodies, rosettes, etc. on the website mikasounds.com, where you can also buy it, they give you the link in the "SHOP" section ! So you don't have to queue before/after the concert, and you can wear them during the concert...
If you want to buy them at the venue, you can go after, there has always been some left after the gigs... Actually, they are quite expansive (i think) so not everybody buy them at the end. But they are nice ! Really...