I've seen the thread inserts you're talking about sold under the name of "Keen-serts". I'd think that places like Napa, Carquest, and certain local suppliers should at least be able to order them.
But, personally- I'd just use a helicoil. I LIKE keen-serts better (they're easier to use, but hard to find)... but helicoils are much easier to find and will do the job just as well- you just have to exercise more caution when installing them. I'm gonna have to disagree with a previous poster who said that helicoils don't restore the original thread strength- if properly installed, a helicoil is actually stronger than the original threads. Also, in cases like yours where longer thread engagement is needed, I've stacked two helicoils in the same hole- one right on top of the other. Works fine. If there isn't room for two helicoil inserts in the hole, you can cut one to length (this requires special care when measuring and cutting, but I've been successful at this several times, including head bolt holes).
Regardless of which thread insert you use, be sure to put locktite on the outer threads of the thread insert- nothing [censored] me off like pulling out a thread insert along with a bolt. Also, liberally oil the inner threads of the insert and the bolt threads so that they won't stick (actually, I usually use antisieze for this... but that probably wouldn't be a good idea for a head bolt. And be sure to install the thread insert (whichever type you use) slightly below flush (one thread or so).
And IF you go with the helicoil option, use a drill bit that is 1/64 smaller than what they reccommend. I can tell you from experience that if you wobble the drill AT ALL using the reccommended bit, then the insert won't have adequate thread engagement. The slightly smaller bit will allow for fuller engagement between the thread insert and the block- and help compensate for drill wobble (which WILL happen unless you can mount that block on a drill press or unless you have a magnetic base drill handy... both of which seem unlikely). If you have trouble starting the tap, then you can enlarge just the first 1/8" to 1/4" of the hole with the recommended size bit.