Usually they are linked up with foreign recruitment companies who have promotions overseas to promote SG education.
You can try advertising online in overseas education forums.
BTW, the money is good but the job is tough. My foreign students are paying their guardians $2k a month (stay in condo, share room with another boy). Those who stay in HDB are paying around $1.8k. Need to include their meals, laundry, coach schoolwork, etc.