Stainless Steel Daggers?

I don't know of any historically accurate Medieval stainless daggers, if you're fussy about "accurate". Using stainless is about as accurate as using plastic, or zinc-aluminium alloy. So a lot of the stainless ones out there have plastic grips, zinc-aluminium alloy guards/pommels etc. And often inaccurate blade grinds. Just search for "stainless medieval dagger" on ebay, Amazon, or just www to find lots. Pick the most accurate one you like. Just be aware that many will not be really functional. The people who want to make historically accurate functional daggers will typically use carbon steels; the people who make ornaments or costume pieces are more likely to use stainless. (People do make good functional stainless daggers, but the ones that come to mind, e.g., the F. semprini Sticking Knife, are not Medieval.)

E.g., www.amazon.com/SZCO-Supplies-203315-Bollock-Dagger/dp/B007XP300M/ (don't know how functional this is)

Note that stainless steel can still rust. It's corrosion-resistant, not corrosion-proof.

The SCA doesn't make rapier swords. Most of the rapier blades used in the SCA aren't stainless.

There's nothing magic about 12". Stainless, done right, can work for full-length swords.

("Carbon steel" can be jargon for steel where the only important alloying element is carbon, and thus excludes alloy steels and stainless steels. Other people use it with looser meaning.)