Dear Estefania Saez Lancellotti , indeed they are expensive. Their chemical composition is difficult to find in most of the commercial products, at least the hydrophobic agent, safety data sheets show just the solvents, Ligroin and abzol, that in plain words are respectively petroleum ether and 1-bromopropane. I only found a seller (Ted Pella) who actually give information about the hydrophobic agent: butadiene rubber. The proportions of each component in their product can be seen in the following safety data sheet:

https://tedpella.com/MSDS_html/22309,%2022311msds.pdf

So it would be possible to prepare a home-made pen, but the chemicals used are potentially quite harmful, and not any container would hold the solution (may be you could refill some old one).

Some alternatives have been previously posted in RG that could be useful to you too, see for example:

https://www.researchgate.net/post/Is-there-an-alternative-for-Dako-hydrophobic-pen

Most permanent markers´s inks are not hydrophobic and therefore they won´t do the job. You need something waxy, so wax, crayons or the like will be able to retain the reactant solutions. I thought about some cosmetics, like lipstics or eye-liners.

Another cheap option that works well is a white pen corrector, they let you draw small circles and evaporation is quite fast due to the use of organic solvents (the ink is usually TiO2 or latex)

Hope this helps. Good luck

Dear Estefania Saez Lancellotti , Uhmm nail polish? Cheap and readily available. Please check the article below for details on how some clever people came up with that 'DIY' solution: https://jprsolutions.info/files/final-file-5f4bb6bdb03225.03037750.pdf

Kind regards, Leo