You can get the code at phpbb.com and add-ons for it at phpbbhacks.com
Hosting you can find for about $45-$55 a year depending on your needs, more for more space, bandwidth, etc... Some hosting companies will even supply you with an option to create a php board easily with features you get with your web space.
Don't forget, domain name about $5- $15 a year depending on what you get (.com, .net, .org, etc...)
If you want a cheap place with lots of space, go with www.uzipp.com . I have been with them for a long time. I'm happy there. They give me a tons of space and bandwidth for only 75 a year, with everything.