Now, those of us who have more than a cursory knowledge of the show know that Magnum's shirt collection went far beyond the Paradise Found Jungle Bird aloha shirt that everyone rocks at Halloween. I've always liked going beyond the obvious and I set myself the daunting goal of trying to identify all of them. Additionally, I'm going to try to see if they're still available today.
I started putting the list together and it was going well until I ran into shirt #6. More on that later, but it made me realize that it's foolish to try to do all this on my own when other Magnum Maniacs are so great at research.
So, with out further delay, here are the first five Magnum shirts (in chronological order):
Shirt #1 (Don't Eat the Snow in Hawaii (1.1))
The Lacoste cotton petit piqué in Navy Blue (Probably colour code 166).

Easily available here.
Shirt #2 (Don't Eat the Snow in Hawaii (1.1))
Paradise Found "Star Orchid" aloha shirt in green rayon.

Unfortunately, this colour variant is discontinued. There's a fantastic and very comprehensive article focused on this shirt (and other Magnum wardrobe bits and pieces) here.
Shirt #3 (Don't Eat the Snow in Hawaii (1.2))
Paradise Found "Mini Anthurium" aloha shirt in blue. Selleck also wore this shirt in a handful of photo shoots to promote the show.



This one can still be sourced from a number of online stores.
Shirt #4 (Don't Eat the Snow in Hawaii (1.2))
Magnum wears this military-style light khaki shirt over his navy Lacoste in the climax scene. I believe it's a copy of a US-issued M-37 officer shirt from WWII.


Hard to find the exact one, but a store here seems to have had them at one point.
Shirt #5 (China Doll (1.3)
Tight-fitting short-sleeved navy blue rugby shirt.


This fitted style with a long set of buttons (for maximum hairy chest exposure) is difficult to source today. But a little digging at rugby supply stores (particularly in the UK) might pay off.