Since people have so much trouble finding this stuff, Dragonbabe has been kind enough to scrounge up some info
on places to buy it. No guarantees, as this is just what she found by searching Yahoo Knowledge-plus in Chinese, but here is info for Taizhong (Taichung):
電話 : 04-5667100
You might need a Mandarin-speaking friend to call first for you to see if they have what you want.
She also says
"There is another store near Banqiao's Wenhua St., near Hansheng W. Rd. (板橋文化路接近漢聲西路口處)", and she
adds that "the weight training stuff is on the second floor"
EDIT: Just found another. There's a chain selling fitness equipment called Sun-Pro (see sun-pro.com.tw) in Taibei, Gaoxiong, Tainan, and Taizhong. The Taibei store is on Nanjing E. Rd. Sec. 4., #21 (ph. 2514-8558). They do have benches and free weights, dumbells, etc.