Seasonal films such as The Christmas Tree can often be hard to find in stores outside of the holiday season, so shopping on the internet might be your best bet.
If you’re looking to buy a particular movie, you can often save time and money by ordering the DVD online. Many online stores such as Amazon offer you the opportunity to buy the item new and wrapped, which might be preferable if you’re buying as a gift for somebody – or used. If the DVD doesn’t work, you can always return it free of charge!
If you’re in the UK or Europe, Play.com offers free delivery, usually very quickly. Amazon does too, but you’ll either need an Amazon Prime account for that, or to be prepared to wait a little longer for delivery of your item using Amazon's free "super saver" delivery, when that is on offer.
You could also try checking eBay for new or pre-owned DVDs.