I found a 'lesser red mutagen' after looting an alghoul, as well.
Found the alghoul in the wrecked town in southeast of Downwarren.
(see appended image)

Mmh maybe this is by design?
I mean, +40% Weapon Power sounds stronger than +600 Vitality. Thus the red mutagens might be harder to obtain.
A question to people completly focusing on signs: Can you actually deal enough damage with them or do you still do much of your damage with the swords? Because if sword damage is your main damage type no matter what you do, red mutagens would be a real boost for everyone.

Signs are quite handy, but due to very small "stamina", you can't cast several signs in row (you can, but very few and in specific order; generally it's impossible to cast until stamina is back to full), and given new alchemy system, it's not really feasible to use postions. I use Aard to throw people away and hope for them to be knocked down, then you'll be able to kill them in one hit, but you no longer can just burn everything to the ground with Igni.
But if lack of red mutagens is by design, I think it's very bad design, judging by new skills system, I think it is.

They need to address the drop rate issue for red mutagens in one these upcoming patches. For every 15-20 green and blue mutagens I get, I get like 0.7 red. This is crazy, I don't want to resort to farming by kill-meditate-kill-meditate, etc.

Same issue - boatloads of blue and green, very few red, don't want to have to farm but probably will since it's probably by design - although it's not like they made the +attack gear sets any harder to get or make, maybe not.

Lesser red mutagen is more or less a rare drop (at least compared to Blue and Green) so far I can confirm that they drop from all Alghouls and Ekimmaras (as for Harpies and Endregas never got any from those in Velen maybe Skellige only).

My route to first Greater red mutagen (combining Lesser red mutagens) in Velen...
Contract: Woodland Beast (2), Codger's Quarry - Velen (4), Quest: Following The Thread (1), Dragonslayer's Grotto - Velen (1), Guarded Treasure Southeast of Devil's Pit - Velen (1). (maybe ran into some other Alghouls along the way)
Note: i'm not saying this is optimal route this is just some location and quest recollection how i got mine (this route also may require some save scumming if luck is not on your side).

P. S. Small offtopic note about Ekimmaras those buggers are so rare that if you don't get Ekimmara hides from them it'll turn out as real nice nut kicker when you'll start to upgrade your bombs.

P.P.S. It would be great help if someone could point to some locations where Alghouls respawn or have a nest (havent't found any yet in Velen)!?

See https://www.gog.com/forum/the_witcher_3_wild_hunt/where_to_get_red_mutagens/post16 attached image. Respawns not after any amount of meditating but after abstinence (just go to some other place and return a few minutes later).

Thanks for the heads up!!! ;)