If we are indeed talking about impulses to be used in a convolution reverb, then it's ok if your reverb dll is missing. Just use Reaper's own ReaVerb and load the impulses into that. If we are talking about a specific Lexicon PCM90 plugin, then I can't find a free one, just a free trial or cracked version of a paid-for plugin. And that is an algorithmic plugin, it does not use impulse responses. Do you have old projects that still have it listed and tries to load it? If so, maybe we can find out what the dll was called.