The handlebars you have in mind are known as “moustache” or “North Road” bars and have become rather voguish in recent years. The most desirable are made in Japan by Nitto and will set you back £50. Sjscycles.co.uk offers a similar bar for £35 — but that is still more than you paid for your bike. You might find something at a cycle jumble, on eBay or on retrobike.co.uk. Otherwise, keep an eye on recycling schemes, your local dump and skips.