If you're just going to do balloons for awhile, look at maybe a colorful vest, several sites carry them but you have some one to sew, you can get any colorful vest made. Same with apron for balloons, to get by for awhile. Approach resturants. some will pay an hourly rate [which include your cost of balloons] you give them out free, some will exchange services and you can work for tips. I've done it for tips and one resturante, I've never charge for balloon animal. Festivals its included in the fee for me to be there. Everybody has their niche, me I perfer to do a shows, only do balloons at birthdays and some festivals, same with face painting.
www.tmyers.com they have balloon cards , I've thought about doing this myself, but using my own pictures, but quick reference to the balloons and limiits request by customer to designs you know.
the legal stuff you'll have to find out on your own; depends on the area and their requirements. Were I am I was told as long as it's a service and I'm not selling merchandise I did not need a business license, but that was here it maybe different where you are. Some festival require you to buy a permit, but again that varies too.