Yes, you can easily buy ammo. But again, most suicides are not really planned. The studies about suicides are quite consistent that only about 10–15% of attempters eventually die by suicide.
That means if you don't have an effective way of ending your life at hand, it's likely that you will not try again successfully.
If you need ammunition, you are not likely to go out and get it. That's inconvenient and even if you go out and want to get it, that takes time in which you might reconsider.
It's worth noting that this is a trend, not a black and white rule. Some people _do_ plan it ahead of time, in some form or another.
E.g. my uncle specifically bought rope and carabiners prior to his suicide, and, apparently, had googled instructions on how to tie a noose several days ahead of the actual thing.
I imagine it comes in waves. And even if one wave just pushes someone to the point that buys ammo, the next has to push it only to load the gun, and the final one just has to push it to pull the trigger a little harder. :/
That means if you don't have an effective way of ending your life at hand, it's likely that you will not try again successfully.
If you need ammunition, you are not likely to go out and get it. That's inconvenient and even if you go out and want to get it, that takes time in which you might reconsider.