From the US? A huge choice of costumes is available at Amazon.com
Apparently, Mario has now appeared in 200 video games, - that's some accomplishment for the little plumber who first appeared in the 8-bit game called Donkey Kong way back in 1981. The first official Mario game was Mario Bros., released a couple of years later in the summer of 1983. The most popular of all, though, was the Super Mario series which first appeared on the Nintendo Entertainment System (NES) in September 1985, and this was the first of a series of sideways-scrolling platforms game featuring the loveable plumber.
If you're looking to dress-up for a gaming event/convention or maybe an event with a retro 80s or 90s theme, then Mario is an excellent option. However, you could also dress-up as one of the other characters from the series of Nintendo games such as Luigi or a super mushroom. The best place to start looking is actually Amazon, because here you'll find a whole range of well-known fancy dress suppliers (inc. Rubies, Henbrandt, etc.) selling their wares at very competitive prices.