Andras,
I don't believe you can get the serial number from the web interface or the console. You will need to get the appliance serial number from the sticker on the device itself.
It's been several years since I initially registered our appliances, so I don't remember what we did, but from reading the online docs, it looks like you should have received some info, possibly an email, that contains the serial numbers for the features you purchased. I think entering those numbers would give you the .slf file.
As a last resort, give Symantec support a call. They should be able to help you. From past experience, I'd suggest you plan on spending quite a bit of time on hold though.
Good Luck