www.hostelworld.com

So you would do yourself a favor if you stop at Suzhou SIP train station rather Suzhou main station. Far less people, much less a mess. Good way to relax. From there, take the bus 115 and go to Jinli Lake. Walk around the lake and enjoy the view and the mood ^^

If you come to visit the old town, stop at Suzhou main station, you can reach the old town with the brand new metro. It will be crowded, unlike Jinli Lake... Hotels, there a truckload of them everywhere. In the SIP area, you can find decent ones for 300 RMB night. Can't recommend one, I live there

or which hostel do most foreigners stay in?

I hear there is one near a Burger King you can try.  Great place to pick up chicks who like white dudes too.  

The Green Garden is the one I checked in a few months ago and I think it was ok for me..

I even got some night service and extra service (not provided by the hotel)

But the only problem was that they were not very good in English language. Had to use my Google Translator. Met a couple from Spain staying next to my room. And yes, their voices disturbed me at night.

i have no idea why you ask this question here,  most of them are foreigners here, as a Chinese you can search it by yourself or ask your own friends, suzhou is not far away from shanghai. 

you can get youth hostel at ping jiang street,its the famous one old street in Suzhou.its cheaper and convinient for you.you can get off at xiangmen or lindlun lu metro station.from both stations,its at walking distance.

always stay in hostels that are close to amenities that you need like train stations , bus stops , big industrial areas ,chemists (drug stores) Burger king, ...