it is hard to find beer yeast where I live. I was wondering if it is possible to just buy a bottle of good beer and use as yeast after bringing it to 25C temperature
Yes (sort of... You can't just warm the bottle up and chuck it in there... It's a little more complicated than that...) but you will need to buy a good quality, bottle conditioned beer (look for sediment in the bottom of the bottle, or the words "bottle conditioned" on the label... Or ask your beer shop...)
Basically, most commercial brewers (particularly the big breweries) will pasteurise and/or centrifuge their beers to remove the yeast from them, and then pressurise with CO2 when bottling; it results in a clearer beer and a more predictable product, but means that there's no yeast to harvest from the bottle...
A bottle conditioned beer will still have active yeast, and can be harvested following the procedures in this article: http://homebrewmanual.com/harvesting-yeast-from-bottles/
Good luck! :)
This is a valid method for collecting and pitching yeast. Many a homebrewer will use a similar method for recreating or deriving specific yeast characteristics in their beers from commercial examples. There are however some things to consider when you go to do this.
The first is the vitality and viability of the yeast at the bottom of those bottles. Yeast are living organisms, or at least, in this case you hope they are. If we consider the conditions of a fully fermented beer, it has a reduced pH, an elevated alcohol content, has low residual (fermentable) sugars and is anaerobic. All these traits help reduce the growth of undesired microbes, but they also stress out yeast cells.
Depending on how long that bottle has been sitting around, and under what conditions, the yeast inside may be on a sliding scale of happy and healthy to dead. Storage of commercial beers, especially in North America, tends to be under less than ideal conditions, consider un-chilled warehouses, with bright lights and even worse if its been imported. Hence to a large extent, the impetus to pasteurize.
So lets say you successfully harvest the yeast from the bottom of the bottle, the entire population will be at various stages of health. Pitching straight into the fermenter of your fresh wort will likely give you a sluggish and slow fermentation that may even stall!
The recommendation would be to create a starter (aerated, low gravity, unhopped wort). Resource. How much to make will depend on style, but a rough estimate is 1 cup of slurry or gallon of starter for five gallons of wort.
Since you don't have a HBS nearby, it would be advisable to harvest your yeast and build your own yeast bank. Rinsing and washing your yeast is an option to keep a population with high viability and vitality, but above all else sanitation is key. Keep yeast cool and under anaerobic conditions if storing (otherwise they will consume their sugar reserves).
If you can get hold of some Coopers Pale Ale or Coopers Sparkling Ale (both Australian beers) you can easily harvest yeast from them. They both have a nice layer of sediment at the bottom. Pour off the top part of the beer (and drink), put the rest (plus the sediment) into a DME starter, wait a few days and you're good to go.