Hello,
Why do you want an interest only loan ?
I would be surprised if you can get one of the retail banks to provide interest only on the entire balance - you might get them to do a part interest only, part amortising loan, if you have a good reason for it though.
Thereafter, the alternative lenders are going to be more expensive (by at least a couple of percent and significantly more, in some instances). Also, I believe that most of them are only providing funds for periods up to 5-years, which may not suit your purpose.