If you were truly hearing the horn relay, your horn switch doesn't sound like the issue there. At that point more likely to be the horn or the ground at the horn, or a short in between the relay & the horn. The intermittent turn signals could also be the flasher relay, not the switch, especially if the turn signals at that point stay on just don't flash.
As for headlight relay, it's a big, about 8-terminal Bosch or Osram transparent cover relay that is indeed expensive and you'd never source from a usual parts store. Hopefully some of those cheaper alternatives will be the culprit!
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