Help Finding Toddler Sweatsuit

My nearly 3 year old son is mad about Agent P from Phineas and Ferb so I thought it would make an easy Halloween costume but I am finding it impossible to find a teal or white sweatsuit that I could dye. I also tried searching for tracksuit if that would help but no luck.

I know I've seen other kids out in sweatpants so they must exist over here. Does anyone know where I might be able to find what I'm looking for? Or even what I should be searching for?

Tesco or Asda clothes section would be my first thought. You could get a white one and then use Dylon dye to dye it in the washing machine. 

Yeah, I think I'll have to dye. Also, should anyone else have this problem in the future, the term I should have been searching is "joggers."

Do people in the US not use the term joggers? Regularly used in BC, AFAIK.