The QuickFire Rapid has a completely standard 87 key (TenKeyLess or TKL) layout, so any standard ANSI set with 87 or more keys will fit just fine. In other words, look around for a set you like and it will almost certainly fit your board.
The QuickFire Pro on the other hand has a non-standard bottom row and it will be almost impossible to find a complete replacement set of keycaps for it.
A few things to keep in mind with keycaps:
1. There are different "profiles", with different keycap heights, surface angles and types of "dent" in the top. Cherry profile is quite low, with different angles per row and "cylindrical" dents. OEM profile is the type that usually comes on a board and is higher than Cherry, but has similar angles and cylindrical dents. DCS is a profile made by a company called Specialised Platics (SP) and it's similar to Cherry profile, but with a steeper angle on the bottom 2 rows. DCS also doesn't have the "crossbar" pieces inside the caps, which is important if you want to add orings (will explain this later). DSA is quite low profile, with no angles on the caps (so any cap can fit on any row) and spherical dents. SA is tall profile with different angles and spherical dents. OEM profile caps can differ slightly between manufacturers / brands.
2. There are different layouts, with the most common being US ANSI. In Europe you will find ISO layout is the more common, it has a taller "Enter" key, often "L" shaped and shorter left shift key with <> key between it and "Z". Then there are different language charactter layouts, for German they use QWERTZ, for French, AZERTY, etc. Nordic keyboards also have keys for ÖÄÅ, etc and symbols in different places. A US ANSI layout board is easiest to find aftermarket caps for by far. There are also two main bottom row layouts, one with 7x spacebar and 1.5x Ctrl and Alt with 1x Winkeys (often referred to as Tsangan, less common) and one with 6.25x spacebar and all 1.25x modifiers (sometimes referred to as Moogle, but it's a lot more common than Tsangan, so is often just called standard ANSI).
3. Keycap materials and thickness. There are 3 main materials that keycaps are made from, ABS, PBT and POM. ABS is the most common and boards usually come with thin ABS caps. Thicker keycaps have a higher quality feel to them and often actually are higher quality. The material is less important than some people here will be inclined to tell you, but it does change the feeling of the caps. ABS gets smooth quite quickly and can feel "sticky" when smooth. PBT wears more slowly and feels "drier". POM also wears slowly but feels more "slick". POM is densest, then PBT then ABS, so POM caps will be heaviest. Heavier keycaps tend to feel higher quality. Well made thick caps in any material are very nice to use.
4. Keycap manufacturers. GMK makes excellent, high quality caps, as does BSP. KBC and TaiHao make reasonable caps, but not up the standard of GMK et al. SP caps fit somewhere in between. Vortex make some good sets and some just okay sets, I think some of their sets are actually made by KBC. Their latest doubleshot PBT sets are apparently quite good, in a profile similar to Cherry. There are other makers, but these are the main ones.
5. Legends. The way the letters are put on the caps. Pad printed are printed onto the surface of the caps and are raised and can wear away relatively quickly. Laser etched legends are set into the surface a bit, laser foamed / printed stick out a touch. These are tougher than pad printed. Dye sublimated (dyesubbed) don't change the surface at all, but are dyed into the material of the cap. Also durable, but can be a little fuzzy. Also can ony be done in a darker colour than the cap itself and often used in ly on PBT material caps. Doubleshot is where they use two different colours of plastic to make the keycap in two stages, first one colour is molded to make the legends, then the main keycap material is molded around that. More expensive to make, but definitely the best type of legends.
So, thick doubleshot or dyseubbed Cherry profile keycaps made by GMK or BSP are VERY good, pad printed thin ABS OEM profile made by some random company.... not so good.