I offer prints on demand at Redbubble, which is my favorite platform (prints and ships from Australia, Europe or North America depending on your shipping destination). I also offer some of my illustrations at INPRNT (prints and ships from the USA), Design by Humans (focused on T-shirts, prints and ships from USA) and TeePublic (focused on T-shirts, prints and ships from Australia, Europe or North America).
Wall Art
FAQ: What print type to choose?
Print quality and look depends on printing technology and type of used surface: smooth, glossy surfaces allow more detailed print and color saturation than rough, matte materials. While glossy finish provides more vivid colors, matte finish gives a more classical, traditional media feel.
Gallery quality prints
Art prints (giclée) archival inks on 100% cotton rag acid free archival paper give the best colors and details among matte prints. Recommended for traditional media, ink and black and white drawings, they also give a more classical look to digital paintings. Available both at Redbubble and INPRNT.
Redbubble offers archival quality photographic prints on Kodak ENDURA Premier Paper. The traditional silver halide prints, exposed using light instead of ink, allow for rich color saturation and contrast, deep blacks and smooth, continuous tone which makes them the best choice for colorful digital paintings. Choose luster finish for brighter, vivid colors (a fine grain texture paper) or pearl like metallic finish for more dimension, deeper colors and darker blacks (glossy paper, examples above and below). Metallic finish also gives interesting effect to ink drawings, giving them warmer tone and less sharp, more dimensional look like they were covered with a transparent surface. White color is less bright, metallic or like a pearl surface, which gives prints slightly darker look than of the luster finish.
INPRNT offers photographic prints in a different technique: printed on a platine rag paper with archival inks.

Photographic print metallic and canvas print (link) at Redbubble.
Other gallery quality prints are: Canvas Prints (matte, Redbubble & INPRNT), Acrylic Prints (glossy with transparent acrylic layer over image) and Metal Prints (glossy) at INPRNT.

Metal print (link) and acrylic print (link) at INPRNT.
Colors & brightness. Preview images at POD sites attempt to reproduce colors and contrast of various print types, as visible on the examples above. Still the final print results might look different from the previews online depending on your screen settings (brightness, blue light filter for eye protection, etc.), so please keep this in mind.
T-shirts

Comparison of white print (transparent black) on blue and black background vs print with black.
For selection of B&W art on T-shirts go to Design by Humans, for different variants to TeePublic, and for more art on different T-shirts types and other products to Redbubble. Some of B&W artworks are available as white print for dark background: all black parts are transparent, so the results are different depending on background color. Regular variants with black lineart are available, too, so make sure to choose "white print" for this effect.