Oh man, I have this package of Crane’s 25 cards which I thought is perfect for you, very thick flat cards, just perfect for dip nibs or anything you want to do and are fairly affordable at $9 for a pack of 25 2.25x6.5” cards.
I did the following using the card, you can see how textured and thick it is.
Unfortunately I just cannot find it for sale anywhere online! The Crane’s website offers cards and envelopes which would fit your request (they have great paper) though I don’t think the card stock is as thick as the one I have. I thought perhaps it could be a Crane’s Colors Card Stock (sold in 25 packs for approximately the price I paid) but those are available in large sizes only. Maybe you can get those and cut em up to size?
I bought these cards from a local stationery shop which is staffed by a rather grumpy lady who marks up everything 10% and tampers with stock so I don’t think there is much help there.
If you want to do a little hunting, here are some pictures of it boxed:
You can see where the shopkeeper lady put her marked up price tag over the original! Arghh.
As for other recommendations, perhaps you may consider getting a boxed envelope and card set? It would be difficult to find them in red but there are some good quality ones with some hints of red, or you can just buy some red card stock.
As for myself, yes I will be sending red Christmas cards too. However, I will only be decorating the address on the front, but my actual cards are pre-calligraphed. Just look at this and you’ll see why I don’t think it needs anything else from me:
Oh good heavens~! <3