Red is a wanted color. I think it's very hard to find a M/L red flap at the boutique at the moment but you can always give it a try.
Personally, I prefer caviar over lambskin, caviar is much more durable IMO.
You can also try to find one on Ebay (sometimes you can find good deals here) but make sure you have it authenticated first before you buy it.
Yes, sometimes the price can be high if it's a popular or new color which is sold out.
If you have a lower budget or if you want a 'cheaper' bag maybe you can look for a vintage Chanel. Sometimes you can find them for very nice prices.
I would personally go for caviar. As mentioned in numerous threads, lambskin scratches, stains, wears etc far too easily. It is true that it is really difficult to find a red m/l flap in the boutique at the moment. Your best bet would be ebay, bonanzle or consignment shops. They do pop up every now and again. If you are not too fussy with the shade of red it shouldn't be too difficult! Good Luck!