For however long it lasts, Five Below currently stocks an “8 Count Fountain Pen” set of $5 (US) rainbow-colored “Made in China” fountain pens that look a lot like the set of rainbow Ooly fountain pens at bookstores and art supply stores. They are not exactly like the Ooly pens: the design of the pen and nib is slightly different and the pen body colors and ink colors are slightly different. The cartridges are narrower (less ink!) than Ooly proprietary cartridges and the 2 cartridges do not swap (i.e. to get more cartridges for your Ooly pens buy them from Ooly.)
Of my set of 8 pens Five Below most wrote after the usual steps with any new fountain pen: wipe the tip of the nib with rubbing alcohol (or hand sanitizer) to remove any protective wax or grease, give the feed a brief bath in warm water with a drop of dish soap or Castile soap, blow through the feed (as if blowing up a balloon) to remove any dust or microscopic plastic shavings, then rise the nib in warm water and shake it dry. Insert the cartridge (directions are on the back of the package: I had to press hard on a firm surface and then remove the cartridge and dip the point of the nib into the cartridge to be sure the plastic seal was really broken – then put the cartridge back in.) One pen out of the 8 was a slow starter so I pressed gently back on the metal nib and dipped it in water: after that it worked as well as the rest.
These pens all came with a little piece of white paperboard to cushion the cartridges inside the pens during shipping. It’s not a part of the pen and can be recycled but it’s an odd touch and a sign of how small these cartridges are. (The back of the package notes that the ink cartridge diameter is “2.6mm”. That would be great if I had any idea of the diameter of a standard pen feed ….) I did get a standard international cartridge (a generic fountain pen ink cartridge) to fit on one of the pens and it wrote without leaking: so once the cartridges that came with the pens run out they can write with ordinary cartridges and there are several brands in rainbows of colors to choose from.
These are also not eyedropper pens: unless you can make the full 16th of an inch hole in the back of the pen watertight there will be no filling the whole body of the pen with bottled ink.
These pens write well enough, have a slight flex in their nibs, take standard cartridges, and are an inexpensive pen for matching the color of your pen to the color of ink, inside (or for trying out no-name inks that might clog a prized, more expensive pen). They are as good as most low-end pens with stamped steel nibs (inexpensive or disposable Pilots, Itoyas, Zebras, Oolys, etc.) and I haven’t yet gotten them to leak. (They do write better if kept point down: better pens should be kept nib up.)
Note: I have no connection with Five Below, I bought these pens on my own at full price, and I owe a thanks to YouTuber SuperRaeDizzle for mentioning that Five Below has a website.