You only need one if you want to play
slide style. There used to be some slide lessons on this site at one time, I am not sure if they were all retired to Kirks' Kirk a Lorange CD. I had a custom 1 of made of leaded glass for 55 bucks Canadian. I also have Kirks' heavy brass slide which I believe is somewhere in the 30 dollar range, you would have to check his order page. Dropped D slides are hard to find, they are shorter than regular slides that cover 6 strings. Dropped D slides only have to cover 4 stings. Regular full sized slides are found in any guitar store,dropped D,not so much.