I have a Canon 1200D and I have a Tokina f2.8 100mm macro lens. I would like to achieve up to 5x magnification. Which bellows should I use?
Using this Extension Calculator it looks like it may be difficult to find a bellows with the amount of extension you need.
To achieve 5:1 magnification with a 100mm lens, you need to have a lens extension of approx 400mm or 15 inches.
You will also need a bellows that has electrical contacts so that you can control the aperture on your Tamron lens. Bellows without electrical contacts are much cheaper, but next to impossible to use, since there is no way to control the aperture on modern AF lenses.
This $759 Novoflex Bellows for Canon EOS has a maximum extension of 5.12 inches and would only allow approx 2.3x magnification with your 100mm lens.
If you really need 5x magnification, your best bet is probably the $1,049 Canon MP-E 65mm 1-5x Macro.