You reasons for wanting to buy contact lenses this way will probably vary depending on whether you want lenses for sight-correction, cosmetic reasons or both.
If you want lenses to correct your vision then it doesn't make all that much sense to be trying to buy contacts without a prescription. How will you know what size lenses to get, and what strength? How do you even know that your eyes are suitable for wearing contact lenses?
It's probably safe to assume, in this situation, that this isn't your first pair of contacts and that you know what strength lenses you need from an old prescription, or perhaps from reading the details printed on your contact lenses box. The bottom line is, you don't have a current prescription but you want to get new lenses without bothering with another eye test.
The second reason you might be wanting to buy contacts without a prescription is that you want cosmetic only lenses. Typically this means colored lenses, or lenses with a design. These lenses provide no sight-correcting powers so you are probably thinking that therefore no prescription is required.
The third reason is a combination of the first two reasons, that is, you want sight-correcting colored lenses. The fact that these lenses are for correcting vision outweighs the fact that they are colored.
You can always find places which will sell you any sort of contact lens without a prescription, even though in many countries it is not allowed.
In Canada, contact lens suppliers are currently permitted to sell colored lenses because such lenses are not, at the moment, classed as medical devices and so do not fall under the same regulations as 'normal' contact lenses. Searching for buy contact lenses canada no prescription should help. Note that this is plano colored lenses, not sight-correcting colored lenses which do need a prescription.
It's always worth pointing out that even if you do find somewhere where you can buy contact lenses online without a prescription, it really does make sense to get your eyes checked and to get an up to date prescription. There's no point in taking chances with your eyes and regular checkups are common sense.