welcome to the board mate!
thats a good question actually - i for one would like to know where to get good vegan gear to wear to the gym.
My current gym fashion is usually wearing the oldest and scruffiest t-shirt around.
I have two t-shirts which have a vegan message which i proudly wear at the gym:
"vegan power" in front and "eat beans, not beings" in the back. My friends made this t-shirt for me about 8 years ago - its getting a bit small now
"89" in the middle and text around it saying "number of animals saved by average person turning vegetarian". Couple people have picked up on that at the gym which is nice.
"XveganX" the tattoo on my left forearm, lol!