I'm sorry bug , I hate to be the bearer of bad news but it's discontinued i'm afraid. I didn't realise that you couldn't get them online anymore or I wouldn't have posted that link. (Sorry) That's where I usually go to when I can't find something in the shops.
I found a page from ebay where someone put out an appeal for some but got no replies. Unfortunately , as you will see , it mentions that it has been discontinued but the ones still in circulation will have a sell by date until the end of the year. I haven't seen then anywhere at all to be honest but I will make a point of looking now and if I see them anywhere , I will come straight back here to update you.
Why don't you fire off a nice little email to Cadbury's and plead your case. They may send you some as a goodwill gesture or give you the bog standard reply about high production costs , low interest in the product , not cost effective , plenty more new exciting products to try blah de blah , pah ! It's always the same with your favourite eh ? They reel you in , tempting you with their choccy wares , get you hooked then snatch it back. Oh the cruelty lol ! Sorry I couldn't be more help and good luck finding some x
http://66.249.93.104/search?q=cache:rav_xBns8e IJ:wantitnow.ebay.co.uk/cadbury-spira_W0QQadid Z5675999287+cadburys+spira&hl=en&ct=clnk&cd=9