A build needs a certain item power, not a certain tier: a 4.3 bow, a 6.1 and an 8.0 all have 1,000. Pick the item and the item power you need, and see every tier, enchantment and quality that reaches it, cheapest first, priced in every city.
Item power from the game's item data: 700 at tier 4 for ordinary gear and 100 more per tier, a little more for artifacts, and 100 per enchantment level; quality adds 20 (Good), 40 (Outstanding), 60 (Excellent) or 100 (Masterpiece). Your specialisations and overcharging add item power on top, the same for every option here. Prices are the cheapest sell order per city and quality from the Albion Online Data Project.