On EBay, Dauntless Hobby in California has a couple of Red Box Richelieu figure sets. I’ve ordered from him MANY times and one of my favorite EBay sellers. His Red Box are listed at $8.99 + $4.95 each, but in his item photos the figures of foot and riders are gray. Only the horses are red. If you order multiples use this seller’s cart system as combined shipping will automatically be applied and save you money.
Judt received the parcel, Musketeers are in grey, Guards of Richelieu in red plastic. Just the men on foot, didn't order the mounted version. You should contact the seller before ordering if the color is important to you. If you're in the UK you could give Kamar a try, if in the US or Canada it's probably cheaper to try a local seller.
From Kamar Figureen, a German seller. Link is in PSR, you can contact them via their website, they've got an online shop and you can pay with paypal. They're really nice and still have some rarer sets. Note that the "command sprue" of the Richelieu set is in grey plastic (as shown by PSR) the rest is in red. PSR also has only grey as color for the Musketeers, don't know if they were ever made in red, perhaps it's a confusion with the Ultima Ratio set?