Don't be sad... I also ran out of mithril and I only have the flare dragon.
If you purchase gems, they should be avaiable as soon as you click the pay button.
I think the gems are never on sale in Europe, I am from Germany and I can't remember if we ever had the oppurtunity to get them cheaper.
However, sometimes you get these iTunes gift cards for like 'pay 20 EUR and get a 25 EUR gift card' if you know what I mean.
Hope I could help you.