have a Canon EOS Rebel XSI and have a Tamron 18-270mm lens for it. Do you know what extender is compatible with this arrangement? I purchased a Canon Extender EF 2X III but it does not fit the lens.
Using a teleconverter on a a Rebel body with a slow zoom lens is not likely to work well, unfortunately.
Autofocus will not work and image quality is going to be poor. You are much better off saving to buy a longer lens.
The Canon teleconverter (TC) has a lens element that extends out from the barrel and so prevents your current lens from mating to it. What that means is that you need a TC that doesn't extend like that. There are a few, but given that you have a Tamron lens, you might consider the Tamron 2x TC as an option. I couldn't find any documentation that said it wouldn't work, but a quick call to their support line should confirm.