Look in the freezers in Indian shops. They come in boxes and inside is a plastic bag with a lot of ice and a few prawns.
They come in different sizes and if you want the largest look for U5 on the box. This means 'under 5 prawns to the pound'. The next smallest is 6/8.
Buy them headless if you can, it will save a little time in preparation.
Always devein them unless you like to get ill

Around here a box is 14quid. For that you get between 9 and 12 prawns.
Make sure they are from india or bangladesh, not asian ones as they are a different variety.
I hope this helps
I hope to post my Tandoori King Prawn sizzler recipe soon. It's as good as what you get in the restaurants and costs a lot less.
KP(V)