I got my one from www.amazon.co.uk and it is similar to the one pictured/linked below...
However, the only downside to the large calculator (there is only one) is that it is not a scientific one and I am yet to find a jumbo sized scientific one (answers on a postcard/tweet/comment please)!
So, it was by luck that I found a solution to this problem this term. The solution came in the guise of one of my department's ITT students who had a Casio calculator that could be used on the IWB! How this is wonderfully possible is that the Casio people have created emulator software that allows you to do on screen to a calculator you'd have bought. This was exactly what I was looking for as so many higher tier students need to be shown, properly, how to use their scientific calculators when performing trig calculations, roots, powers etc etc.
So, I got myself one of these and have loved using it ever since. It now has a permanent home on my desktop at school and I can quickly call the calculator up whenever needed to show my students exactly what they'll need to press on their calculators to perform the calculations required. This has really helped my teaching of trigonometry this term to my top set Y11s when revising 'basic trig' (SOHCAHTOA), the sine and cosine rule and the area of any triangle using 1/2absinC.
I have found in the past that students often type in calculations incorrectly on their calculators assuming that the calculator is doing exactly what they need. For example...
With this question, where you're asked to work out the length of (in this case) the opposite side. Students would need to do sine of the given angle multiplied by the length of the hypotenuse. So, naturally the kids type in sin, 3, 0, x, 1, 0, =. However, as you can see from my emulator calculator this would work out sine of 300, not sine 30, then multiplied by 10. The student needs to close the bracket on the automatically opened bracket.
This then allows me to show the students, and discuss with them, why this is wrong much, much easier than it would be to 'talk' them through it. I can then show on the IWB what they need to type in.
There have been so many good discussions that have come from me using the emulator on my IWB and going through examples with my students. I've had a lot of 'what does that button do, sir?' type questions that allow me to then show them things they can do with their calculators that they didn't previously know, all in an attempt to make it easier for them to pick up marks in their examinations and avoid losing marks through incorrect assumptions on what is being 'done' on their calculators. One of my favourite recent discussions was on the 'sexagesimal to decimal' button, see http://goo.gl/YVtEwy for more info!
You can look into the Casio emulators by clicking on the link below. I'm sure there are others available, but as my students, on the whole, have Casio calculators, I've chosen this one as it's the same as they have in their school bag/hand!