To buy altcoin then the first thing that you have to have bitcoin or eth, then you can buy on the exchange such as yobit, huobi, poloniex. But it also depends on your country, as I see, the United States and Japan are the two countries with the most exchange.
True, most usually this is the case. I faced the similar problem; of having several accounts at different platforms and transferring between them, just to get hold of my altcoin (from fiat currency). So, I started working on a project last year and we just released our P2P platform, with focus on supporting trade between a wide range of cryptos with any fiat and any payment method. It's similar to localbitcoins, but will support many different cryptos. Feel free to have a look at coinected.io - Currently we have no fees, as we just started.