What you are referring to are known as supplementary lenses. There are cheap sets of these widely available which should be avoided as they are a complete waste of money. The cheap ones consist of a single glass element and are prone to awful colour fringing.
More expensive supplementary lenses, commonly known as achromats consist of 2 or more glas elements in each lens and can give very good results but they aren't cheap. Canon make 2 achromats in various sizes to fit different filter threads, these are available in 2 powers 250D and 500D (not to be confused with camera bodies that have the same numbers). Other brands are available but make sure they are labelled as achromatic.
Supplementary lenses can achieve some good results but they only work over a limited distance and may not be adequate or convenient for some small subjects and a 1/2 inch flower is probably asking a bit much, they are a poor substitute for a proper macro lens.
