For cheap reputable brands, perhaps you can look into the Polygon line of bicycles. They can be found at Rodalink
If your purpose for the bicycle is just for transport and marketing, any less than $100 bicycle found at your local bicycle shop will do. Or if you are not that fussy, Carrefour and Giant had a variety of bicycle you can look at.
If you are looking to get a mountain bike for offroad riding, I would suggest you put a reasonable budget, and spend some time to shop around. While we have quite a number of bicycle shops in Singapore, each shop will only carry a few specific brands. Some brands are also only exclusively available at 1 shop.
Perhaps you should state what is your main purpose for the bicycle. Then it'll be easier to make more suitable suggestion. Also be more specific in your budget. To me, a budget entry level mountain bicycle costs less $500.