Where can I find a large heart shaped punch?

Have you tried Hobby Craft or The Range? I got a couple, smaller than the one you want though, but they both have a good range of punches.

Try on here...https://www.craftsulove.co.uk/punches.htm they have billions of punches

The problem with a standard punch is that you may only be able to punch the heart up a certain way, and as you say, it may not be able to reach to the correct position. I'm not sure if this one would be big enough - but it allows you to position it anywhere:

http://www.beautifulpapercrafts.co.uk/craft-punches-c17/martha-stewart-punches-c45/martha-stewart-punch-all-over-the-page-studded-heart-p423?utm_source=froogle&utm_medium=ppc&utm_term=martha-stewart-punch-all-over-the-page-studded-heart&utm_campaign=froogle#570

If you want to stamp on the dark card, you can do so if you use a metallic light (ie silver) coloured ink pad - it would show up on the card. If you want to emboss the stamp design it will show up even better. You need to stamp the image, dust over embossing powder then melt it with a heat gun. There are lots of instructions on Youtube about how to do this, and embossing powder and gun may be less than the cost of a large punch.

HTH - give me a shout if you want any more info!

Sorry, had forgotten all about posting this!

ATB - That's the punch I came across too, but it's still smaller than I would like and pricey. I think I would rather use the heart stamp I already have that I used on my invitations so that the OOS ties in with them, so I think stamping sounds like the way forward! I'll have a mooch down the local craft shop and see what they have to offer. Thanks for the fab advice :-)