I heard that it is very important to keep your guitar in place that's not too humid or too dry, so I was wondering if there where any dehumidifiers/humidifiers that could fit in your guitar case that I could buy somewhere.
I have a couple, and they seem to do the trick. Check your local music store?

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Bags of dessicant will absorb moisture, while Planet Waves makes a small humidifier for acoustic guitars. I'm unaware of one product that performs both functions.