Near the stadium on Nguyen Thai Hoc street are a lot of sports stores. There is one on the 'triangular' corner that seems to have equipment. (think it's Hung Vuong Street)

Hi there!
You can buy a yoga mat at my Zenith Yoga Studio on 31A Van Mieu Str.,it's by the Tample of Literature;my phone no: 0904356561, the cost is $22 and a yoga mat bag for $15.
Take care!
Hari om!
Marzena

You can get one from the Nike shop (1st floor)at Vincom Tower. Costs around 400,000 VND. Have fun!