Spirit binding.

[Anova]

Honestly, any place with a fast spawn or a loop with enemies around lv.45 is good. One of the most popular spots for spiritbind is Urth's Gift in South Shroud.

As for the kind of materia you want to get, that will vary depending on the marketboards. Most crafter and gatherer materia will sell well, as will det and crit. If you have a 50 goldsmith available, keep some electrum ingots and jewels around for a quick hq craft. HQ items sb faster, as does melding junk materia to them (Fire, Wind, etc). You will want ilv.45 or ilv.55 items to spiritbind as those will yield rank3 and rank4 materia.

[Niwashi]

You will want ilv.45 or ilv.55 items to spiritbind as those will yield rank3 and rank4 materia.

If you're doing it for profit, this can also vary according to the current state of the market board. For instance, last time I checked, on Coeurl, rank 3 materia (of all types) was actually selling for far less than rank 1 or 2. (I'm guessing too many people were trying to SB the level 45 stuff everyone talks about, so the market just got too flooded with it for a while. It's been a while since I checked, though, so I'm not sure if that's still the case or not.)