As Hammer already said, you can buy a shell account (usually a few dollars a month) and set up a bnc (that stands for bouncer) on that shell account.
A search on
Google.com can give you some sites who review shell account providers.
Also, there are some networks that automatically provide some type of host masking for its users. Webnet/Webchat would be one of those networks. In that case, it would be the usermode +x.
However, just keep in mind that this type of service is not provided by mIRC itself, so this isn't really the best place to ask. All we can do is to refer you elsewhere.
