Farming materials for Machina Mail and Machina Blast takes a lot of screws which are a fairly rare drop. For each person to get from Leather to Machina Blast+3 it takes:

80 Screws, 4 Generators, 40 El Aldo Steel, 4 Genuine Liquified Oil, and 40 Refined Steel.

Most of the materials can be easily obtained on the frozen Islands North of Cureo once you have the boat. The only enemies on the islands are Thabit, Thabitor, and Thabitest.

El Aldo Steel can be pretty easily farmed at the cave East of Chasmus. After harvesting, go into Chasmus to reset the cave materials. The distance between the cave and Chasmus is short, although I found the little picnic area too easy to stumble into.

For the Refined Steel, I like the mine West of Lecom. There's two resource sparkles, one is only Refined Steel, and the other is Silver Ingot / Refined Steel.

The Machina Mail isn't actually that great compared to other armor you will have at the time you get the boat, but going up to Machina Blast is a big improvement (D. Power:340 HP Recovery+13%). Machina Blast+3 is the best armor (blacksmith says it can't be improved). It gives D. Power 569 HP Recovery+13%.

It is possible to have the armor for all 3 characters. Bell and Aulin aren't meant to wear it, so it must be upgraded while equiped and never taken off. Because of this, it's wise to collect all the materials for one of them to upgrade to at least Machina Blast before putting on the leather armor.

To purchase leather armor go to the dock area in Lecom and talk to the guy with swords laying in front of him.

If you need the tomes to manufacture Machina armors talk to the hidden guy in the southwest corner of Dandong. As soon as you enter Dandong walk West all the way and then South.