Book 18 establishes that there exist Andalites who SUPPORT the Yeerks! Wouldn't such Andalites arrange for Andalites to become infested?
I rather thought that was what he was attempting to do in that very book. Note he stuns his bridge crew, he doesn't kill them. He's not just delivering Andalite ground attack vehicles to the enemy, he's not just punching a hole in his own force's ground forces, he's supplying the enemy with something so valuable they'll give him . . . well, theoretically whatever he asked for.
However two things occur to me as simple reasons you never see other Andalite controllers. The first that occurs to me is that most of the Yeerks who would have an opportunity to infest an Andalite might not want to. Bear in mind that they
hate Andalites as much as Andalites hate them. Visser 3 was the exception: he was
obsessed with them, he wanted to infest one ever since he was young but there does seem to be at least a few of his fellow controllers (Iniss 226 comes to mind) who even seem to consider him half-Andalite, albeit when they're irritated at him. Many of the Yeerks probably don't want to
be Andalites.
It just seems to me that this is a war where both sides neither ask nor grant quarter. Notice the Andalites are never shown to have freed Hork-Bajir colonies for those thousands of Hork-Bajir they captured and freed . . . because they didn't free them. Partially because they probably don't care but also because this is a war that has gotten to the point where both sides hate each other so much, they Hork-Bajir is just collateral damage and a small price to pay for killing the Yeerk that infests it. For the Yeerks surrender isn't an option, they'll be starved (rank and file Yeerks would think this even
if Andalites might keep them alive as prisoners which I'm not convinced they would) for the Andalites surrender isn't an option, they'll be infested.
The second reason that occurs to me is that the best and only time to take an Andalite for yourself and ensure that you'll be the one who gets to infest them is probably in battle. Not a large scale pitched battle but probably a smaller skirmish, you want as few people on both sides as possible to limit the likelihood that you'll get friendly fire after you've secured your new host, right? Now this also means you have less support, might not even be telling others what you're planning . . . now two things: first there's your old host, as we see with Visser 3 in the Hork-Bajir chronicles his host did not just sit idle once he was out of his head. How many voluntary Hork-Bajir hosts do you suppose there are? Now assume you get into the Andalite's body you've now got one angry Hork-Bajir (because they're the type of Controller most likely to encounter and subdue an Andalite, Taxxons would likely just eat them, we never really hear of any other sorts being used on the front lines) to rip you to shreds in the five to ten minutes that it's established (in book 6) that it takes for a Yeerk to establish full control for the first time in a new host. That Hork-Bajir will simply kill you.
In short (I know, too late) for a Yeerk to infest an Andalite you need the desire (most won't have it) and the opportunity. Given Andalites' ability to self-terminate and the rarity of successfully subduing one to boot . . . tricky.
A rank and file Yeerk would never pull it off, they would need a small conspiracy: friends to support the attempt both to restrain the original host and help capture the Andalite himself, few Yeerks would trust one another enough to hatch such a plan and then actually make it work. Once your trusted buddy gets shredded your plan is ruined, you're fighting for your own life and the stun setting on your dracon beam suddenly doesn't seem as sensible as the kill setting.
