Just like normal
The kingdom building rules do not change the other methods available to buy/sell equipment and magic items. Increasing your cities' purchase limit (Base value) will simply make those things available all the time (or at 75% chance), just like any other city. Oleg, being a merchant/trader, may still do his trading with Brevoy and take a few weeks to bring whatever item is requested from him.
Whatever is above the city's base value, may still be purchased at GM discretion (from Settlements):
The base value of a settlement is used to determine what magic items may easily be purchased there. There is a 75% chance that any item of that value or lower can be found for sale in the settlement with little effort. The base value of a new settlement is 0 gp. Certain buildings (such as a Market or Tavern) increase a settlement’s base value. a settlement’s base value can never increase above the values listed in Table: Settlement Size and Base Value (except under special circumstances decided by the GM).
This GM Fiat is exactly considering specific scenarios like special requests, or paying things in advance so they can be sent to you, or even request for something to be crafted especially for you.
Note that, while a normal settlement has an initial base value of 0 gp, any city you build using the kingdom building rules is already considered a Small Town and has an initial base value of 500 gp instead.
Not only that but once your kingdom is established, you may also negotiate trade routes with other kingdoms, using Trade Edicts. While those edicts have a specific mechanic that affects the kingdom's economy, nothing prevents you from making requests to those traders to bring specific goods from other kingdoms.
Finally, I strongly recommend the third-party books from Legendary Games (Ultimate Ruleship, War and Battle), they are written by the same author of those rules originally (from Kingmaker), Jason Nelson, and they expand the kingdom building rules and the mass combat rules in several ways, including fixing a few hiccups with the published rules.