I reuse them even after storing raw meat in them. Fill them with soap and water and microwave for 30 seconds, or until the blood is thoroughly cooked. Rinse.
I did the math a long time ago. It’s cheaper to buy high-quality ziplock bags and reuse them many times. But if you’re just going to use a single (cheap) bag twice, you might as well use two bags. (From a purely cost point of view)
If you’re looking for them in the grocery store, they’ll be in the kitchen section (at Carrefour, they’re near the pots). They’re called 保鮮袋 if you’re looking for ones made for food or 連鎖袋 if you’re looking for ones made for storing small things (not food).