If you have one nearby the most obvious choice is the post office. They have been selling stamps since stamps became a thing (see the history of stamps here) and they will be happy to help to make sure you have enough stamps for what you are sending. If you are one of the unlucky people who do not have a post office nearby however there are still a number of convenient options for you to choose from. For a more in depth look at Where to Buy Stamps. If you are looking for larger numbers of stamps (10+) then you have a lot more options. There are plenty of large chains across the country that stock postage stamps as well as envelopes etc.
IMO on PC comes with some great features which are mostly same over every platform you are using. This is an instant and convenient messenger that allows you to send and receive messages across the whole globe with a great ease and also it is very easy to handle and use on any platform.
VidmateVidMate allows you to convert music video into audio files, download your favourite movies in HD format non-stop from over 200 supported platforms, and all of these for free.