For an MSP fee, most items of higher tier can be fused (synthesized) with packs of items of previous tiers, but only if the object in question is required by a unit. The MSP fee goes up if fusion of additional lower-tier items is necessary, i.e. if they can be in turn fused under their own requirements.
Aside from being commonly dropped while playing / raiding levels in Attack!, items of this tier can be purchased in the Shop in stacks of one for 1,500 MSP. They can also be acquired randomly via the MSP Crank in stacks of eight.
Aside from being commonly dropped while playing / raiding levels in Attack!, items of this tier can be purchased in the Shop and Extra Shop in stacks of one for 3,000 MSP or two for 6,000 MSP respectively. They can also be acquired randomly via the MSP Crank in stacks of three.
As seen in the table below, purchasing Bronze items directly is much more efficient than purchasing Iron items and fusing them into Bronze - about the same as fusing Silver items from purchased Bronze items. Only surplus Iron items acquired as loot drops should be fused.
Items of this tier are the only ones not obtainable directly in combat. Instead, one can purchase these in the Rare Shop for 3,000 Medals per item (although this is an extremely poor use of Medals). They can also be bought in stacks of one in Battle and Team Battle Shops, Guild Shop, and P.O.W. Shop for 10,000 pieces of the corresponding currency.
As seen in the table below, items that require Platinum items for fusion offer substantially better value when purchased. However, as every Platinum item is a considerable investment it is not advisable to purchase best-value items in advance (unless a specific unit would require it). Furthermore, if there is a steady supply of Silver and Gold items dropped as loot and time is not an issue, it is advisable to postpone the purchase - and even if Platinum items are immediately useful, it may even be worth waiting depending on when the next Coin Item Sale is scheduled (as these lower the price in the Battle / Guild / P.O.W. shops to 7,000 of the relevant currencies).