If you're very lucky, it's the battery cable connections are dirty/loose, and COULD even be a bad battery.
The most common bad part in situations like this is the starter.
If you're not sure, you better find out before buying parts.
You wont save by buying a starter and asking a shop to install it for you. They don't like that, as you are cutting out some of their profit margin. They will make it up by charging you more on labor. Shops will not warranty a part carried in. Others will not install a carried in part.
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