Slayer is a nice way to get charms while gaining some xp too.
Some specific crimson charm droppers: Rock lobsters(burst/barrage obv), waterfiends, dust devils, black demons, greaters demons, dagannoths.
You shouldn't hunt for blues and should just pick them up while you get them but if you really want them I would go cannon wallasalkis at waterbirth. This is really expensive just like rock lobsters though.
Also remember to do the charm minigame once a week. You can go for example go to waterfiends for 40 minutes with it and gain around 150 crimsons.