It's hard to be too specific, because one never knows what will be available in your area. However, this might point you in the right direction.
At that price, you might be able to get a bottle of Vintage Port, but prices for this cover a huge spectrum, and your local stockist might only have more expensive Vintage Ports.
If not, then look on the label for the words "Late Bottled Vintage". This is slightly different from Vintage Port, but only because it is matured in the barrel, instead of the bottle. This is a more economical process, and so Late Bottled Vintage port is less expensive.
Do you have a local supplier like Oddbins? They are very good.
Also, for a Christmas treat, it might be nice to avoid Port that says "Bottled for [name of supermarket]". Instead, look for a bottle imported directly from one of the big Port houses, such as:
Fonseca
Dow
Warre's
Graham's
Taylor's
A Late Bottled Vintage from any of these will be a delight, and should be within your budget.
Bear in mind that Vintage and Late Bottled Vintage Port will have a sediment. It will need to be stood still for 24 hours, and then decanted, before you can drink it. This ritual is part of the pleasure.
If by any chance you don't have a decanter, then choose a Tawny Port, again from one of the companies named above.
Good luck.