Depends on the source, but yes, there are a few that are hard to get elsewhere, such as speed or luck sources. Guard sources can be morphed from the much easier Spinel or whatever silver grey things around the brown soil near mideel, power sources can be morphed from the heavy tanks near the gongaga reactor (and is the only enemy to spawn there). I usually don't even bother with magic and spirit sources, as they are so slow to get from morphing those big HP enemies.

3-4 of those huge insect like things in mideel will be faster if you have slash all, as it should be possible to kill them all in one swift attack there. It's also safer and doesn't require you to heal hp/mp as often, which also takes a bit of time.