Farm for the Cesti in Wanderer's Palace. Prices for these items vary greatly from server to server. For the materia, buying them will probably save you a lot of time because spiritbonding is never guaranteed t yirld what you want/need. If the price you mentioned (20K) is for both materia, then grab them. If that's for each, check the sales history to see if that's an inflated price for your server or the regular one. They used to sell for a lot less when I played on Exodus. Sometimes prices shoot up after a rich individual buys up all the cheap ones, but after a few days things should be back to normal.

For the actual forging and melding, i'd recommend getting to know some people who can do it for free. FC mates, usually. For example, my FC is tiny but we all have one or more crafting classes at 50 so all crafts and melds are basically free (just gotta gather/purchase the mats).