Selling 'Free Cain Velasquez' Shirts

Fighters across the UFC are throwing their support behind Cain Valesquez following his arrest on Monday ... with Derek Brunson even making "Free Cain Valesquez" shirts to try to help pay for the MMA legend's legal fees.

Brunson announced the creation of the tees and the sale on his social media page on Wednesday ... explaining 50% of the profits would go toward helping Valesquez foot his legal bills.

The shirts -- which come in white, blue, red, grey and green -- have a $27.99 price tag.

Brunson actually modeled the shirt when announcing the sale -- posting a photo of him in the tee with the caption, "We support you @cainmma."

As we reported, Cain has been hit 10 charges -- including felony…