You could probably use some RTV (get the kind for thermostat housings) in the mean time as a temporary fix, but you should replace your radiator asap unless you like replacing head gaskets... I have never heard of just replacing the plastic end tank.
A replacement radiator is fairly cheap, only like $50 to $75 from the junkyard, and around $100-150ish for a new OEM-style replacement (CSF radiators are okay, I just put one in mine)
GL
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