The Final and First Question
is to ask "what purpose does this serve in the story"?
When you think about approaches, think about the following upside/downside questions. If both of them make sense to answer in the scenario, even in a metaphorical sense, then the upside and downside are clear enough and that approach can probably come into play.
Approaches have more overlap with each other than skills do in Fate Core. It's entirely possible for one situation to lend itself to multiple avenues of approach, but probably not all of them.
- Careful: Why do you have to take your time? Why can't you take too long?
- Clever: What are you focusing on? What else can't you afford to ignore?
- Flashy: Who are you showing off for? What don't you want them to see?
- Forceful: What are you trying to push? What aren't you trying to break?
- Quick: What are you racing against? What can't you do to win?
- Sneaky: What has to stay hidden? What will they find first?
This of course assumes that what you're trying to do is appropriately scoped for the results of creating an advantage - getting a temporary aspect into play with maybe a couple of invokes on it. Like if you're prepping for a tower heist in the evening, and you go walkabout in the Infinity Fair looking for something that'll give you a little edge.
If what you're trying to find is more important than that, if the last extant crystal key to Dark Stobolous's planet-cracker has somehow wound up sold to some shop in the Infinity Fair and you have to find it before the goon squads do, that's more of a contest, or even a conflict, depending on how you want to run it.
Rolling Your Own
And then there's bolting on other things, such as player membership in guilds to provide them with those kinds of resources. You might consider giving the guild some notional stats - the least you can get away with is its overall quality as a guild for that character, and any special aptitudes (+2) or ineptitudes (-2) at getting stuff. Then ask yourself, what happens if the roll fails? What cost is there for the guild to impose on the player?
Or even make guild membership into a stunt, that like, once per session, you can hit up the guild's requisitions department and they'll have got you any two pieces of specialized gear you want, as represented by aspects with one free invoke on each.
The Final and First Question
is to ask "what purpose does this serve in the story"? If it's just the transient focus of a scene, like anything could be, making a roll with an approach is the best way to address it. If it's to secure something vital to the plot, that you can't proceed without, consider zooming in into a contest or a conflict, and keep in mind that anything in the world, like bureaucracy or the mean streets, can itself be a character. If it's something you find yourself doing so regularly you wish there were special rules for it, well, make some.