This should be a go-to resource for existing and new users to look for regular to extended Desk Mats / Desk Pads.
AliExpress - Holy grail of Desk Mats with prints ranging custom, anime, PUBG, CS, anime, etc. [Tread with caution, especially with custom mats with rubber base - number of bad quality reports]
Amazon - A large resource to buy mouse pads and desk mats. Advantage - shipping and replacement. [suggested by @stevesie]
Doyu Studio - Cherry MX / Mechanical Switch Desk Pads with stitched edges.
Flash Quark - Cherry MX / Mechanical Switch Desk mats with stitched edges.
InkedGaming - Custom pads with loads of size options. [suggested by @donutcat]
Keyclack - Topre switch Mousepads [hat tip @AugustCanaille]
KPRepublic - Cherry MX / Mechanical Switch Desk Pads without stitched edges.
Massdrop - Game of patience. Enter at your own risk.
MousePads Cool - Large to desk size mousepads [h/t @caleb]
NovelKeys - Quality deskpads to match your Keysets.
PCGaming Race - Glorious Desk Mats ranging up to XXXL.
Rama Works - Canvas Deskmat for 60% and 65% keyboards [h/t @sK0pe]
Shiden Kai - Japanese Artisan Desk mats, mostly targeted at eSports.
Switch Source - Cherry MX / Mechanical Switch Desk Pads with stitched edges.
Zap Cables - Custom designed, funky looking desk mats.
Gaming Desk Mats
ASUS - ROG family desk mats.
Cooler Master - RGB glow and other desk mats as well as mouse pads.
Corsair - RGB glow, hard surface, cloth, and even one with wireless charging!
Cougar Gaming - Balanced, Speed, and Control desk mats for gamers.
Dell Alienware - Just one extended mouse pad for Alienware fans.
Dream Machines - Speed and control with microfibre surface and rubber base.
HyperX - Stitched edge and textured with red accents and colors.
Logitech - Large desk mat with rubber base.
Mionix - Deskmats with food symbols.
Razer - Variety of control and speed mouse pads along with aluminum and RGB glow ones.
Roccat - Precision Cloth, Silicone surface and double-sided mouse pads for gamers.
Steel Series - RGB glow, Cloth, and Textured Cloth desk mats, highly recommended by pro gamers and enthusiasts.
Tt eSPORTS - RGB Glow in Hard and Cloth editions, other mouse pads.
Turtle Beach - For gamers.
Zowie - Cloth and Low-friction deskmats, recommended for gaming.
Please recommend more stores that offer Desk Mats / Desk Pads.