A set of transparent straps often come with a convertible or multiway bra or you can often find them in the accessories section at lingerie stores. You will need to use them with a convertible bra that has detachable straps so they have somewhere to hook into.
They aren’t as invisible as the name suggests, if you wear them with something strapless you won’t fool anyone. What they are good for is if the straps you normally have are a bad colour match for what you are wearing and you want the glimpses that people see of them to be less noticeable. Another use is if what you are wearing is lace or sheer, you will probably still be able to see them through it as they catch the light, but again they will be much less noticeable than coloured straps.