The online retailer for lovers T shirt design is Design By Humans

Design By Humans was born in 2007 in Irvine, California, USA with the simple vision of bringing artists from around the globe together to give them a platform to produce wearable art that tells a story, strengthens humanity and inspires life through creativity and design.
according to wiki: Design By Humans is a world-wide online shopping store specializing in all-over screen printing and on-demand custom printed apparel, phone cases, and art prints. Their apparel offerings include t-shirts, tank tops, and sweatshirts. The company specializes in hosting design competitions and creating a community around graphic art and t shirt design. With a world-wide community of over 15,000 artists and graphic designers

Design by human is creative community of T-Shirt design lovers. From shopping for new graphic tees, organizing design contests and communicating fashion trends; we are a society of artists, fashion gurus and expressive humans. Design human is a passion for brilliantly designed tees, detailed illustration and a strong appreciation for all things creative. Street Fashion Store offers 35K different pieces of artwork which includes t-shirts, sweatshirts, racerbacks, phone cases, art prints, and much more.

Each garment that we produce starts with a story that is told by an artist from Spain, Australia, Europe, the United States, or one of over 110 countries from our design community that submit hundreds of T shirt printing designs daily.
Through our Design By Humans Collective initiative, we’ve emboldened epic artists and given them a unique platform to share their t-shirt designs through their own online storefront.

Come to Design for humans you get more benefit from: 

• Best discounts, hot deals, coupon and sale off 
• Over 15,000 diverse international artists from all around the world
• Nearly 35,000 designs
• Free domestic shipping on all apparel orders over $75
• International shipping to 180 countries

Source: Design By Humans