World Wide Arms have both Mk II and Mk IV for sale, however are a little on the pricey side (albeit cheaper than the link above)
Obsolete Rifles & Muskets
www.worldwidearms.com
There are still bargains to be had, I think I paid around the £700 mark for my Mk IV but I had to clean a lot of gunk off of it and replace the striker spring (easy job and cheap), however the bore is mint. My Mk I/II was only £600, but there isn't much rifling left on it.
If you are on facebook, there's a really good M-H group on there and it's often worth posting some pictures on there with the question, as there's an awful lot of Khyber Pass fakes out there, and if you don't know your M-H's it's easy to get a dodgy one.
Another option is to get one from IMA in the States. The exchange rate isn't too bad at the moment, so you're looking around £900 for a Mk II cleaned and restored (cheaper if you want to do the restoration yourself), however you'll need to pay import tax on this. I know a couple of people who have imported them from IMA and they think that the rifles are great value and worth the wait.
What is a "cleaned & Complete" antique gun? These are antique Firearms that have been broken down, cleaned and carefully restored to be in a condition as close as possible to the day they were made. <
www.ima-usa.com