"Converter"?
So far as I can tell, you're connecting analog outputs from your mixer (they kinda hafta be analog outputs, since nobody puts digital outputs on 1/4" jacks that I'm aware of) to analog inputs on your soundcard, right? The only "converter" you need is cables with a 1/4" plug at one end and an RCA plug at the other.*
I expect your new soundcard has digital inputs too, which also probably use RCA jacks. If you really wanted to, you could buy a converter and put it in between your mixer and the soundcard. But, since a significant part of what you're buying the soundcard for is its ability to take a bunch of analog inputs and convert them to digital (i.e. its AD converters), I've got to figure you don't want to buy another set of AD converters. It's sort of like buying a converter to use with your converters.
Now, for the intricate subtleties of "converting" 1/4" to RCA: the easiest solution is to use cables that come that way. You can even make them yourself, if you're so inclined. If, for some obscure reason, you insist on using cables with the same kind of connectors at each end, you can buy adapters. I would only suggest that buying an RCA-RCA cable and RCA(F)-1/4(M) adapters is preferable to a 1/4"-1/4" cable and 1/4(F)-RCA(M) adapters, because the adapters are smaller, cuter and put less strain on the jacks.
*Okay, you also need to worry a tiny bit about balancing and levels (you know, +4 and -10).