They show up something like 1/5000 hacks. More often the higher level the portal you're hacking. There also appears to be some relationship between usage and getting them: people never seem to acquire more than about 20-some unless they start stashing them in a capsule, but if you use them regularly you seem to get more. That may be confirmation bias, but a lot of people have made similar observations. (Likewise, people that blow up a lot of stuff seem to get slightly more bursters, and people who deploy buttloads of shields seem to get more shields.)