Both stadia and stadiums are accepted plurals of stadium. Neither is right or wrong, but stadiums is far more common. This is the case throughout the English-speaking world, and it has been for several decades.
English-speakers are not required to know the rules of Latin grammar, and most Latin-derived words with long histories in English are now pluralized in the English manner. We do still prefer some Latin plurals by convention, however, but stadia is not one of them. Besides, stadia has its own meanings unrelated to stadium (i.e., a telescopic instrument used to measure distances, plus several related definitions), so keeping it separate might be useful.
Stadia as the plural of stadiums seems most widespread in Indian-English publications such as these:
Regardless of this, pre-pubescent and pre-menopausal audiences will kill for tickets and standing-room at the stadia. [Times of India]
Director Sports Pargat Singh said there would be ten games for which matches would be held at various stadia across Punjab. [Indian Express]
It also appears occasionally elsewhere—for example:
The truth is that you could say the same of pro football stadia anywhere. [NJ.com]
The Middle East country was too small, it has just one major city and no big football stadia. [Yorkshire Post]
But stadiums, as it appears below, prevails throughout most of the English-speaking world:
In the shadow of one of the most iconic stadiums in football lies a depressing picture of dereliction. [Telegraph]
There are even more impressive efforts at energy-saving in sports stadiums around the world. [National Geographic]
She campaigned for female fans to be allowed to enter stadiums. [Guardian]
The issue of public subsidies for corporate-owned stadiums and arenas has been a source of contentious debate nationwide for years. [Los Angeles Times]