Various sources to try (in order).
Your dept (I assume you a grad student?) normally giving a talk/poster is a requirement to get funding.
Your research council
College (oxbridge by any chance ?)
Then you can see if the conference has any grants - if it is being run by a well funded industry/international body it might have.
There are also a few general charities that award grants, your university should have a directory/someone that knows about them.
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