Mechanical keyboard don't have super thick heavy duty power traces, so basically any cheap $25 dollar 30 watt iron from Lowe's or other hardware store should do the trick for your needs.

When looking for solder, look for flux core or rosin core solder, practically all electronics grade solder is hollow and has flux/rosin already inside the solder.

For larger projects where you're dealing with large power traces and/or large wires, you might want to invest in a more professional temperature adjustable soldering station, and additional flux.

What old flux I've been using (I admit I haven't done all that much soldering in years after I quit that job, chemicals aren't exactly healthy yo)..

NC-559-ASM-TPF(UV)

But hey, if you want some cheap flux, just go cut some bark off of a pine tree and let the sap run for a day or two. The original soldering flux was pine sap.

Good luck fully cleaning that shit off though, pine sap is super sticky..