Best place for crimsons is waterfiends but i only recommend going if you have 68 summon already, they drop crimsons at an exciting rate lol, i can get about 200-300 a trip plus a nice amount of noted goodies to sell as well, such as water orbs and snape grass ETC (I'd save water orbs for 80 summon though when you can use them as seconds to make hydra pouches.
Another good place for crimsons are Nechs if you have 80 slay, or greater demons, they're not as good as waterfiends but they still have a good percentage of crimsons for drop rates

Here's a link to another website (runescape.wikia.com) with some information about crimson charms and the monsters with best drops
http://runescape.wikia.com/wiki/Crimson_charm