We call them linings when they are internal doors, they are not made the same as external door frames. The width is chosen to match the finished thickness of your door. If you have a 3-story hime or for other reasons need a fire door it is all heavier, thicker and a bit more expensive.
A professional chippy will fit a door and lining quicker and better than a handyman or general builder. It might take a day (say £130) to fit one, but if you are doing a number of them in one go, it gets quicker because there is less to-ing and fro-ing.
The cheapest possible door is a hollow flush door. Don't buy one. It is made of air held in place with a think skin of hardboard.
Some door are called "woodgrain" which means they have an embossed fake finish supposed to resemble the grain found on an old railway sleeper. They are horrible. Don't buy one.
Remember you will need hinges, knobs or handles, and locks or latches. There are some rules for these depending on age and style of house and door, which I will tell you later. You will find several thousand on IronmongeryDirect.com at cheaper prices that poncy retailers sell them.
The first thing is to find a good chippy by personal recommendation and look at an example of his work. Carry a 10p piece with you and slide it all round the door. It should fit the gap between door and lining all round. If he's any good he will know the local door merchants for quality and value.
Here are some examples. Measure your doors so you can select doors of the size you want. Like all building materials, they are sold in millimetres. Yours will be 35mm thick, unless fire doors or exceptional quality, when they will be 44mm.
www.wickes.co.uk/internaldoors
www.wickes.co.uk/Products/c/1000000?q=%3AtopSellers%3Acategory%3A1001165%3AMaterial%3AHardwood%3AMaterial%3ASoftwood%3AMaterial%3APine%3AMaterial%3AOak&show=All&perPage=15
www.premdor.co.uk/
www.ironmongerydirect.co.uk/search?SearchText=door+handles