Keep in mind the following recommendations:
- For the basic and intricate imbuement items, you can kill the creatures on any level 15+ character, and one at a time. Some of the spawns mentioned below have a lot of creatures in tight spaces, so level 30+ is recommended for mages.
- For the powerful imbuement items, you'll need a high level character to kill grim reapers, or a blocker instead. Paladins are more suited for this task.
In my opinion the Upper Spike (for levels 25-49) is the best and most accesible spawn, I suggest you check out any YT video made by Gudii 
You can also try any of the Ankrahmun hidden tombs: the Stone, Tarpit and Peninsula Tombs have a lot of mummies. You'll need a scarab coin to go through the flame teleport, and you can hunt in the upper floors with level 30+ mages.
If you want to do quick sessions logging in/out in a low level character like me, there's a small spawn with 5 mummies deep below the Green Claw swamp (Venore) where you can nicely sit!
Gloom wolf caves like Jantle suggested. However, you'll need to check if the mini world change is active. To do so, check the board in the Adventurer's Guild for this message: "Poachers have slaughtered nearly all wild animals north of the Green Claw Swamp. But vengeful spirits show up there now!".
There's another cave located inside the Green Claw swamp (Venore) called the Tainted Caves where you can find gloom wolves without needing to wait for the world change, albeit in a lower density.
You can use stealth rings to avoid incoming damage in lower level characters.
The best spawn is the underground dungeon in Yalahar's Cemetery Quarter, below the nightmares, where you'll only find grim reapers. There's another dungeon deep below Drefia too, where you'll do a lot of walking in between a much wider area and you'll also find gravediggers and spectres, not just grim reapers.