Attaching a bottle with metal collars used to be the only way to do. Any decent 20-year-old bike shop should have them in stock. However, they are not exactly nice looking and will only fit standard tubing, as that was the norm in 1960, 1970 and early 1980.

There is one trick that works as well and is probably even better looking: zip ties. Put a piece of double-stick tape under the base(s) of your bottle cage (to prevent scratches and rotation), and attach it with 2 zip ties. Tighten them, and you will have a relatively nice-looking set of collars. Easy to do with a nearly white or nearly black bike; harder to do with other colours, although they might be painted.

Regards,