Is it possible to get a visa for mainland China in Hong Kong, when not travelling with a travelcompany?

And can I go anywhere in China with that visa, or am I restricted to Guangdong province?

Yes, you can. you can go anywhere except Tibet with Chinese visa.
please check the link below you will find whether you are eligible to enter without visa: http://ask.jongo.com/qa/detail.jng?id=2974

all the best in China

sunsky travel does china visas pretty cheap. depending on where you're from a 3 month visa to all of china will probably cost you around hk$600 and can be processed in 1 day. sunsky - hk 3426 9813

Yes, you can obtain visa in the custom