Is it possible to buy powdered colors that are used for celebrating Holi after the said event and if so, where?
Same question goes for those color powder flares, color powder canons and paper hot-air balloons that are so unbelievably fun to use!
Holi supplies is a seasonal industry, similar to how fireworks are a seasonal industry based around Diwali. Most commercially available Holi dyes will be hard to find outside the festival season, although you should also be aware that most of them are unsafe due to presence of heavy metals such as lead and mercury! Yes it looks fun, but many of them can cause allergic reactions or even if not, carrying around colours with heavy metals in them for months isn't a good idea! The safer dyes are called by a different name - gulaal (pronounced 'goo-la-al'). Not sure whether you'd find these off-season.
Some households make their own natural Holi colours / dyes - here's a list of some colours that you can make. Perhaps this is closest solution you have to what you want.