I often see oval chocolate discs used by professional chefs on TV to melt for chocolate couverture, etc. I've generally used bar chocolate, but think using these could be useful for measuring out small quantities. Do you know what brand of chocolate discs is used (they all seem to use the same one) and where it could be sourced?
Chocolate couverture is generally good-quality chocolate with a high level of cocoa solids. Nigella tends to prefer to cook with chocolate that contains 70% cocoa solds, though in the US you may have to search out a European brand or use a high-end brand of bittersweet chocolate that has a minimum of 62% cocoa solids. Please note that if you are using either type of chocolate for melting and coating and you want a very shiny finish then you will need to go through a special temperature controlled process called "tempering".
The oval chocolate discs that you may have seen are usually referred to as chocolate wafers. These are much more common in the US than in the UK. They may be marginally easier to measure out than cutting up a bar of chocolate, though you will need scales to measure both. The main advantage is that they are a uniform size, so they melt evenly. In the US the Guittard brand of wafers is the easiest to find on-line (see suggested link below), and you may also find them in some cake decorating stores. Don't confuse them with candy melts which are a very different product.
In the UK you could buy high quality chocolate chips to use for melting, though good quality chocolate bars are easy to buy in most supermarkets so we would suggest these are more economical to use and it is fairly easy to measure out and roughly chop the amount needed for a recipe.