www.alternate.de is very quick, althogh I don't know if they guarantee next-day delivery.
You could look at
www.amazon.de. I don't know about their RAM prices, but usually they are amazingly cheap and also offer free shipping if your order is higher than 20 Euro. But next-day delivery might cost extra.
From my experience a usual online order without extra charge needs two days, not one.
Bye,
Thomas
