Diorskin Nude in the colours that end with 0 such as 020 have neutral undertones. I'd suspect that either 020 or 030 would be similar to NC20/NC25, just not as yellow.
You could also have a look at Mac Face and Body foundation. They seem to run less yellow - I use N1 which is probably around NC20, but it doesn't seem yellow on me. It is very sheer and you might need to apply 2 layers, but it does set nicely on the skin.
Clarins also seem less yellow than a lot of other brands. I have the Everlasting Foundation which is very matte and medium/full coverage in 107 Beige, but it doesn't seem massively yellow to me.
Laura Mercier oil free tinted moisturiser (and probably their other foundations/tinted moisturisers) also do not seem massively yellow.
Estee Lauder should have some foundations in neutral tones. I think they are generally quite generous with samples so it might be worth having a look there too.
I personally find the Bobbi Brown foundations to be very yellow - but she does believe that 99% of the population are yellow toned and therefore doesn't have any neutral or cold toned foundations.
Clinique are also either too yellow or too pink for me.
Make sure that you get a sample or that you get the foundation applied on your face (Mac doesn't give out samples) before buying as you might find that some foundations will oxidize and get darker/more yellow/orange after a while on the skin.