For one off and short term stuff, you can use a company called Event Helper. They issue policies that are short term and you can add additional insureds as well. You can generally write up a policy within a day. It is nice to have a full policy that you can generate and file within an hour or two. The issue is that for what you get they are expensive. Typically $160 + per day or event. I have a policy that is REALLY cheap in the grand scale of things, but my business model is a bit different too. It is best to try and find a local company that offers general liability policies. The cost is dependant upon several factors. The one big thing is what the gross income of the company is. If you only draw in $50k a year it will probably be cheaper than you think it could be. If you draw in $500k + a year I would bet that you pay roughly $5k a year in insurance. It seems to me that most seem to set right around 10% of gross business income. I.E. $100k a year in gross receipts will be roughly $1k + a year in insurance costs.
As with most things insurance related, past and history will get lower rates. Basically, you pay more your first year than you will your second and consecutive years once you have built a history. I got a quote for $25 a month about a month ago....... Yeah, that's a bit too good to be true. I pay quite a bit more than that, but I also have two guys I can call in which we all know each other by name. The company that sent me the quote was called Next Insurance and they do have policies for sound industry applications! I just don't trust in the lowest bidder for something that is such a serious thing. If you want a full real policy check out Next Insurance and see what they can do for you. My fear is that they are cheap for a reason.
