A part of my shed door snapped is it better to construct one or buy one which works out cheaper and where can I buy?

when you say shed do you mean a wooden garden shed or brick built outhouse shed? Ive recently bought painted and fitted one on a brick shed. Went for a stronger framed ledged and braced type. If its a timber shed then a bog standard ledged and braced door will probably do.
Reynolds Timber in Birkenhead can make them, or Howells DIY wallasey or Kel Reclaimed in bebington. Cost around £70. I ordered mine off internet made to measure Cottage pine interiors in oswestry as it was around £75 which included delivery.
Band Q or similar do a kit for about £35 but you have to put it together and not had good reviews.

Door £75
New T hinges (wickes £6 pair)
New galv suffolk latch ( £6)
Paint £20
Labour Paint and fix anywhere between £50 and £75

It's a wooden garden shed.