The only easily available place to find the right kind of trichoderma for your situation is in a product by a company called plant success and the product is called
"great white" it has a trichoderma variant that is essentially the same. It's about the most expensive beneficial fungi cocktail that you can come across on the market these days at about $10 an ounce if you buy the 8oz container, but if you know what your doing, you can culture your own from it moving forward. I think it is way over priced, but also without a doubt the best single solution available today. It's upside is it has Myco fungi, trich, and other beneficial fungi that will make truly very happy, hearty, and well inoculated trees. It's overkill for bonsai in most cases as you can get VAM beneficial fungi for a hell of a lot cheaper than great white, but for your case in your situation even as preventative maintenance i think the great white will be worth its weight in gold. The only thing to keep in mind after application is do NOT feed for 3-4 weeks, then moving forward STAY AWAY from high P fertilizers .
Your other option is to buy a really expensive microscope, and some books, a shovel, sheers, and a bucket and go foraging for it. But it would take you a year or more to find what you need to get started.