I know what you mean. One idea you might consider is grabbing a beta outside your normal circles. For example, I don't know if you ever bother with FF.net (it's kind of lost its charm for me), but they have the option to advertise yourself as being willing to beta, and betas can set up profiles for what fandoms and content they're willing to handle. So if FF.net is outside your normal pool of readers, that might be one place to look up a beta without spoiling anyone you already have.