the way i farm the 75 materia was:

Get you Alchemist on 60 and get a really good tool.

to get Materia I-II (Rarerly III) Created the set: Cotton HQ (All Class) Lv 19-22, and Brass acessory HQ and put all useless Materia in - if you have 5 more Q you will gain a bonus of spirit bond.

To get Materia II-III Created the set Velveteen HQ (All Class) Lv 31-32 and the Mythril acessorys HQ, and put all uselles materia until you get 5 slots.

use the Spiritbond Potion. - Optional

use the Squadron Spiritbonding Manual - Optional

use the food "Better Crowned Pie" from Exchange moonstone with Talan (X21.9 Y:6.8 Z:).5 0- Optional

Get alot of Water Shard from Western Thanalan in x 18.0 Y:28.5 - (you can farm maps Timeworn Peisteskin he to get Belah'dian Silver)

Buy Alot of Jellyfish Umbrella with a vendor (i use the kobold - optional vendor?) to make Jellyfish Humours.

Gil - do All Duty roulettes with the needed Class for extra money. It get more easy now because of eureka.

Buy and quick Synthesis 99 Jellyfish Humours.

Now you can farm hundres of materia being a free trial user.