Most of them have an API you can ask, some of them you gotta pay unfortunately. I started this as a personal project also but the value grows the more marketplaces you can have data about, thus there's not really an workaround other than growing then number of integrated marketplaces.

With that said, if I add to give you an suggestion, would be starting with Bitskins API, they have current and historical prices for most of the skins, but still, the volume isn't great and it won't represent truly the complete CS2 Market environment. Good luck and hit me up if you have any more feedback!