Magic is extremely common in Eberron, but there are relatively few high level NPCs out there. This means that powerful magic items are usually found, not created. If you happen to have the Enchant Magic Item ritual and you have the money to spare for the arcane reagents, you are perfectly free to create your own magic items within the limits of the ritual and the associated Artificer and Dragonmark feats.
Magic items of level 5 and below are available at standard market prices and are in effectively limitless supply. Specialist antiquity and arms dealers are able to source items up to level 10, but these are relics of past civilizations or scavenged from the fields of battle left over from the Last War. The prices for such in the source-books already take into account rarity, so your concern becomes finding someone who sells the kind of item you want.
Just as in Heroic tier play, your PCs will be awarded Cannith Tokens by their respective Dragonmarked houses instead of randomly assigned magic items in treasure parcels. Cannith Tokens directly represent magic item levels. The tokens are handed to an artificer at a Cannith enclave in exchange for a magic item of your choosing (up to a maximum item level of Level + 4). The Tokens can be accumulated and combined as long as you do not exceed the maximum.
During each level of play, each PC will be given a Cannith Token that represents an item of the party level plus two, i.e., at some point while the party is level 11, each PC will be given a Level 13 Cannith Token.
In practice, each PC will end up with roughly one free item level worth of magic loot over the course of several levels due to rounding. As the GM (me) is somewhat forgetful, I make up for this by throwing loot at the party at odd intervals in the hope that it makes up for my lapses of memory!
The purpose of this is to remove some of the suspension of disbelief required when a monster just happens to have the item you had on your wish-list!