Well some companies can offer you a free server, say 1GB and in return ask for either donations or advertising. Usually the people who get a sponsor have a good number of YouTube subscribers (1000+) or are a popular mod/plugin dev.
You may think that this is a herculean task but I have seen other less popular minecrafters get servers, so you always have a chance.
For a better shot I suggest you message the hosts directly, not just posting on the forums. Talking to each of them individually shows determination and drive, all great qualities for a potential advertiser.