Although it takes a lot more searching to pull out the office-appropriate shoes from the pickings at Forever 21, they do have a large selection of shoes that are 100% synthetic.
Some, such as the pair to the right, could be worn to a casual office, and are available for a song ($14.80).
Delias also has a selection of polyurethane shoes in the $35-50 range, if you can dig past the bright patterns and sneakers. For example, these kitten heels, these bronze flats, these black T-Strap low heels, and these ballet flats in black or pink. Many of these are notable since the PU material is more weather resistant than canvas, satin, or other fabrics.
They also have unique T-strap flats in snakeskin-print black or a metalic lavender (the latter perhaps more appropriate for evening) and carry two colors of poly-satin Rocket Dog flats. Keep your eyes peeled for Rocket Dog; although Delia's selection of this brand is limited, they do appear to produce a wide range of vegan shoe options.