Buying a replacement I/O shield for a Atom mini-itx mother board is probably not a great deal more difficult than than buying a regular one,but more than likely unless you order it through the store you bought it from or an authorised re-seller in the UK you will have to do an online purchase from abroad.
How to go about it
Find out the exact model of your motherboard & who makes it (if you built it yourself) or contact your system manufacturer (if you purchased a complete build) ie.Dell,Compaq etc. (Looking at your manual is your easiest option,looking at your purchase receipt or tax invioce or looking for the printed model No. directly on your motherboard or if you are lazy download a utility (a program) called cpu-z from here http://www.cpuid.com/cpuz.php , but be warned, itx is not a standard build & it may not be recognised by this program.
Here's a screenshot of what to look for [IMG]http://img269.imageshack.us/img269/1338/cpuz3.png[/IMG] ,this is an example of what my system looks like so expect that there will be variances when you run it.
For more info about this program refer to the home page listed above or do a search of my previous posts on this forum.
Find out the Part No. for the I/O shield you need by contacting the sales or tech support section of your motherboard or system manufacturer specifying that it must be the itx version for your motherboard & not the standard one (atx if I remember rightly) via their website or telephone if they give you a local option. (website is cheaper but may take a couple of days).
Ask the manufacturer if you can order a replacement part from them (may take a couple of weeks if it's an international supplier) or do they have a local reseller in your area.
If you have a Dell,Compaq etc they usually have a program that is called something like "system analizer" that scans your system over the internet & tells you what you've got & if that works for you I'd suggest you bookmark that page for future reference, then it just a matter of going to their spare parts section & hunting around till you find it & if that fails contacting their sales support section.
A few of things worth knowing,
1. ebay is nearly always the cheapest place to get things like I/O shields ( they usually come with the driver disk for your motherboard when you order it)
2. Companies like Dell etc often use third party motherboards like Asus,Foxconn,Gigabyte etc. that if they haven't customised the motherboard may be worth looking on the third parties home webpage.
3. This is the most common solution to any hardware forum enquiry ,if you know it, "POST YOUR SPECS!" (that means model No.'s),the reason why is there are millions of different models of hardware worldwide,which means people will give you well meaning but vague & non-specific advice,if you post your specs people will give you answers specific to your exact problem or as close as they can manage.
4. If you cannot find something yourself ,accurate descriptions means people can search the web/forums & in other places you would never think to look ie.if you can find 10-20 results in google ,odds are the more people who look at your post will find at least that many results multiplied by in some cases quite a few more.
5. This case is a good example,this post is in the processors forum,but if you try a couple of different forums,try under case modding,motherboards or at an overclockers forum(don't overclock your hardware,you'll void your warranty) but they tend to be very knowledgeable about most aspects of hardware whereas this predominately a forum for a processor & chipset manufacturer frequented by enthusiests & a few techies.
6. Try looking for hardware reviews in google about your hardware ,a few good places are AnandTech,Toms Hardware or xBit labs but there are many more.
Finally, I strongly do not recommend this,but you may be able to install the motherboard without an I/O shield but it will get dust in it & there will be definately airflow issues in the case which will cause excessive temperature readings with your hardware but post on a case modding forum about that,I'm not qualified to give you the kind information about this kind of situation that you need.
All said & done I wish you the best of luck.