Everyone should start here with the Stardew Valley Wiki Player’s Guide to Getting Mods. Another excellent resource is this Youtube guide by Salmence.

Unfortunately, a lot of mods end up dead over time since they take so much effort to maintain and, often times, for very little pay. If you like a mod, you should try to tip the creator to avoid this in the future!

However, community members will sometimes get permission from mods creators to maintain and update them. If you google the mod’s name, there is a chance someone will have an updated link in a forum somewhere.