hai ma pai = Seahorse brand
seahorse brand is from Hong Kong, made of foam, quality not so good, doesn't last long, after sometime it sack.
there are normal spring and pocket spring for the spring mattresses, as for tension (how firm the mattress is), its better to lie on the mattress to feel it yourself, whether you find the mattress too 'hard' or too 'soft'
as what KPO recommended
you can get cheap mattress at
Rolands furniture, there's one @
Blk 501 West Coast Drive # 01-300
tel: 7730322