All my sportswear is from either Nike or Adidas, you can easily find them in Singapore but most of the designs are pretty normal as compared to the ones you can get overseas.
In terms of price, Johor Premium Outlet has got the cheapest sports wear. Usual price for a sports brand is about $55-$65. I got it at $16 from there!
Jalan Premium Outlets
Indahpura, 81000 Kulai
Johor Darul Takzim
I stocked up quite a lot of sports wear and shoes during my last two trips from Taiwan too. You can find them easily at any night market. Can’t remember the price but it’s cheaper than Singapore for sure. Do keep a lookout for fake ones, it’s better to visit their official store!
Otherwise you can try out Mustafa, I remember that the price is cheaper there also.
For online, I will suggest ASOS!