This is a very common expression in English that can mean all of the following: that is to say (that is is really just the short form of that is to say), in other words, or more exactly, or even of course. The Collins English Dictionary, the link to which I provided above, defines it as follows:
You use that is to say or that's to say to indicate that you are about to express the same idea more clearly or precisely.
We could rewrite your example like this:
It's not a problem to rely on a powerful weapon. If you can find one, of course.
It's not a problem to rely on a powerful weapon. That is to say, if you can find one.