I haven't seen a good guide. But, if you do not have a gathering and/or crafting class above 35, you can still make a healthy amount of gil by:
Farming Raptor Skins, Toad Skins, and Boar hides. They are used in most high level synths for wearable gear and for gear used to make materia.
Oak Logs are dropped by a few mobs, which, while not that great for gil, can still be decent.
Alumen is another ingredient that sells rapidly, and is more about the volume than price.
Leves, while boring to repeat constantly, two people can consistantly do leves to build up for the 100k leve, then rinse and repeat. While slow, you can still pull in about 1 million gil a week with dedicated play.
Other than that, you can try and game the market. Buy cheap level 41+ gear that people put up that can convert to good materia, than spirit bond it, and convert to materia. While you will end up with more items worth less, you should be able to get a few good materia (especially if you know how the conversion works) and make a profit.
My recommendation here would be to verify the number of different types of materia an item can turn into, and choose the item that has the least number of different types, and/or the most number of types that make money.
Than you get exp (if you have class above 40 but below 50), and you can kill things that drop items (such as the hides) or gil. You can also go for SH runs for coffers and get gear from the coffers for conversion while you are converting.