One can purchase solder paste from various stores online and offline. Some of those stores are SparkFun Electronics, Amazon, Ebay, DigiKey and SMT Solder Paste.

Colin C. Johnson has written:

'Solder paste technology' -- subject(s): Solder pastes

You can use a paste flux and heat (as from a soldering gun) to reattach the solder.

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To facilitate the soldering process, a chemical material called flux can be used to remove oxidation from the surface, keep the air from oxidizing the surfaces during soldering, and to aid in the flow of solder through the joint. Fluxes can be found within the solder (cored solder) or separately in paste form.