The device you saw was probably a "solar projection eyepiece". You can use any eyepiece for this, in theory, though eyepieces with cemented lenses can heat up in the sun and the glass can crack.
Personally I wouldn't feel comfortable using one of those with my telescope.
You can use solar film specially designed for looking at the sun, such as Baader's "AstroSolar Safety Film" which comes in a variety of sizes.
The link above is for just the film, but if you want to use it with a telescope then they also sell the film mounted in a frame which clips onto the front of your telescope.
Further down the line, if you get more into solar observation, then you may want to look into buying a Herschel Wedge which filters the light after it's passed through the telescope, so shows more detail.