If you re in the UK, visit the IPO (Intellectual Property) site. They provide a lot of useful free help and advice on this. They also run free events around the country. We went to one in London and it was really useful.Good luck!
Full search at the US Patent Trademark office online is free.Is it more thorough? Depends on what search words you use.Similar products? Yes, you can make an IMPROVEMENT of an existing product and patent it.Yes you can file a patent without a prototype but it s more difficult to convince a manufacturer to license it without a prototype.You may want to file a Provisional Patent first (marks the date at USPTO and gives you 1 yr to see if it s marketable) and see if there is any interest with manufactures before filing the actual patent.Of course you can always manufacture yourself - then you ll need investors. That s another ball of wax altogether.