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The QFS and Pok3r both have standard layouts but I think the Quickfire does not. A standard layout, especially ANSI, makes it so much easier to find replacement keycap sets.
First thing is to decide a budget. Cheap sets come in at around $25 (for a full set that will suit a standard layout ANSI keyboard). The most expensive sets cost $300 and up, usually for rare or unusual colours in limited runs.
Most sets from GMK will be around $100 to $150, and from JTKeycaps will be about half that price.
Tai Hao is a good low end keycap maker, and their keycaps are really quite reasonable for what you pay. They have some half decent PBT sets too.
Or perhaps the colour is more important to you than price - many people are looking for a specific colour set to match their other hardware or decor.
In either case looking through
interest checks and
group buys is a good place to start.
If you don't mind a wait, Massdrop (massdrop.com) has some good deals on keycap sets.
And then there are a bunch of online stores that stock all sorts of keycap sets, although depending on popularity you might find it difficult to locate the set you want in stock.
