New business help

[CleverCatty]

I've got potentially a good new business idea but would like to do a business plan for it.

Best place to get these or start them?

Also - my business, I potentially would like to ask someone else who has a similar business for advice as a mentor - would anyone advise this or not a good idea?

Most new businesses don't need a full business plan initially unless you need funding.
As to whether you get advice is whether you can afford it, need it and if you have a chamber of commerce nearby you may be able to get some help for free. Always do background checks on someone giving business advice. I come across a lot of people fresh out of uni, never run a business in their life, but they are advising. There are also lots of people who do run their own businesses, but their business has never really taken off. Again, not qualified to tell you how to run a business!