There isn't a sponsor letter for a business VISA, if by that you mean an Entrepass.
The whole point of an Entrepass is that you are creating a new business, or you are a business partner of an existing business who is going to take an active role in the business in Singapore.
The primary requirements for an Entrepass visa are a comprehensive business plan, and filling out all the blanks on the application form which ask for projected turnover, staffing estimates, etc. You will also need a CV and it is wise to submit educational and other professional credentials.
The only place that another company gets involved is in the guarantee. If you can find a company willing to guarantee that they will cover the $3000, then all they need to do is also sign the applicaiton; otherwise, you will need to see about getting a bond.
See
http://www.mom.gov.sg/MOM/CDA/0,1858,12 ... --,00.html
for details.