So You Want to Master Synthsis?
Here's what I used to finish all the rank 1 requirements using just 3x of each item:
Clothing: Wave Print Side-slit Tunic (From Malcolm) (You could get Tipping Suits from rafting, too)
Shields: Vales Shields (From Taunes) (If you have Heater Shields, those work also)
Weapons: Crown Ice Wands (From most magic shops) (You could use Phoenix Fire Wands or Crystal Lightning Wands also and if you have a lot of, say, Demonic Lances or something, those are 10k NPC value also)
Headgear: Starry Wizard Hats (From Gilmore) (Broad Brimmed Feather Hats are 10k NPC value and relatively easy to tailor, too)
Gloves: Silver Teany Gloves (From Fleta) (You could make Valencia Cross Line Plate Gauntlets as an alternative if you have the materials)
Shoes: Fabien Greaves (From Fleta) (The cheapest blacksmithed alternative is Dustin Silver Knight Greaves as far as I know)
Note: This is only taking into account equipment always sold at an NPC. For best savings you could buy everything on a a Wednesday for the shop discounts, preferably the wands from Kirine or Castanea for further discount, depending on alignment. Sometimes you'll get a lower than normal durability resulting item, which means it'll actually be less than the NPC resale value you need to fulfill the requirement. For example Ice Crown Wands are normally 11 durability and have an NPC resale value of 5700g, but might sometimes result in a 10 durability wand through synthesis that only has an NPC resale value of about 4800g.
It's possible you could get away with not using 3 of the same item, but the chances of getting the same item in return might not be as high.