Do you mean downloading as in like acquiring them is hard to do from say archive sources or, forum threads?
Or installing mods for Minecraft Beta 1.6.6? I think you mean the latter, so what you need to know is that mod instllations for beta versions is not the same as current Minecraft mod installs.
You need to understand META-INF and those old ways compared to now where you just put it in the folder after installing Forge or Fabric.
I don't understand how to get mods working on old versions so best to ask on the Modification Station discord https://modification-station.net/ or find Youtubers that have coverage of older versions (either old videos or recent and those with experience with that time/that way of installing mods).