There are many to choose from. If you are looking to spend a relatively small amount of money, look on Ebay and many perfectly passable versions can be bought for 10 or 20 dollars. Most at this price bracket are Chinese made. If you are looking to buy used the best model around is the Silva Alba Windwatch. There are two versions so if you want the electronic barometer function be a little careful as some versions are anemometer only but look identical. The barometer model is identified for the UK as the "Alba" version But they were made also by Silva for Minox, Nexus and several other companies. Pay around 30 dollars and find them on Ebay. I have found them to be the most reliable and accurate pocket anemometer/barometer.
For my meteorological studies I use a JDC Skywatch. You would pay around your top end but they are bulletproof, waterproof, very accurate and will last for many years. They are slightly larger though being around 6" tall, so more something for your backpack than a pocket. Again, several versions that look similar. JDC Skywatch are Swiss made.
Hope this helps
