SEE BELOW
In the 1970s, we used Kodak Wratten #87 filters, which could be bought in 10cm by 10cm sheets for not much money, but Kodak stopped making them. The story was that then new environemntal regulations had rendered manufacture of the necessary dye too expensive for the market. (#87 filters may be back; I haven't checked in years.)
The alternative was actually better than Wratten #87 filters, which were expensive, fragile, and tended to bleach under the light.
What we used instead was a tripack of ordinary theater-light filters, such as those made by Rosco. These filters are used in front of a
1000-watt lamp, and only ~4% of that is visible light. Of necsssity, all theater-light filters pass infrared, to avoid combustion.So, what one does is to stack three complementary filters, such that all visible light is blocked, allowing only infrared to pass.
What you want are generically called "plastic gels" in the stage lighting trade, and are made of dyed plastic film, not gelatin. The Rosco product is "Roscolux". I don't recall the exact colors we used, but it wasn't hard to figure out. One just got (for $15) a sample book, and tried combinations out.
There are a number of competitors as well.
There are glass filters that pass IR and block visible, probably made by Schott and perhaps Hoya, but these were too expensive for us. Theater filters are something like $6 for a 20" by 24" sheet, and widely available.
Joe Gwinn