You can buy 'mha light novels' at many large bookstores like Barnes & Noble. They usually have a good selection of popular light novels, including those from the My Hero Academia series.
I'd suggest starting from Volume 25 of the manga. It aligns well with where season 5 ends and takes the story forward smoothly. There might be a few differences in pacing compared to the anime, but it'll give you a seamless continuation.
The MHA manga can be accessed on various platforms. For instance, Shonen Jump's official website might have it. Sometimes, local libraries also offer digital access to popular manga. Just do a bit of searching to find the one that suits you best.
After season 6 of MHA, a good place to start the manga is around chapter 315. This chapter continues the story arc and keeps you engaged without feeling lost.
Several characters have met their demise in the MHA manga, but it's a bit complex to list them all precisely. Some of the notable ones include [mention specific characters if known].
I think it's hard to say for sure. There are a few characters who could potentially be the traitor, and it's all part of the mystery and suspense of the story.