I've looked plenty and I think I can handle the issues with the controller though it would be better if I could just replace it. I guess I could also get a quality battery pack (Turnigy?) and take apart the cells, but I'd prefer to buy matched loose cells from a known-good brand (Sony, Panasonic). I haven't seen anyone selling matched loose cells though.
For a while I was scrapping dead laptop batteries to build an 18650 pack for my esk8 build. I ended up building a battery validation rig with a Turnigy battery charger that charges the battery to 4.2V, drains it to 3.8V, and charges it back up to 4.2V to figure out its true storage rating. Then I sorted them by manufacturer, model, and storage capacity, with a recycling bin for anything below 80% of their rated capacity, open cells, and shorted cells.
It's a lot of work, but the validation was all automated via the charger. The pack I ended up with had healthy cells all within 10% of each other in terms of capacity.
By "matched", do you mean binned by the vendor for capacity and internal resistance? I don't know of anybody doing that, but dealers like Illumn might be willing to sell you multiples with the same lot number. I'd be surprised if most OEMs do more than that when building packs.
You can get a cheap analyzing slot charger ($30-40) and do it yourself. That won't be quite as accurate as a big expensive industrial battery tester, but it will be accurate enough to make safe battery packs for consumer electronics.
