There are a few option available, and the first one is to buy it directly from the Steem Monsters Market. You pretty much just log in to Steem Monsters, click on the "Market" tab in the menu, find the card you want, then pay for it by using SteemConnect. Make sure you browse the correct card set before you purchase any cards, since Alpha and Beta editions of the same cards cannot be combined into one.
As you can expect, all the cards on the market is put there by people who wants to sell them for that price, so the price and value of the cards will always fluctuate a bit.
Once you have bought a card, wait a few seconds and it should show up in your Collection.
Peakmonsters MarketIf you want to buy many cards at the same time, using the Peakmonsters market could be a good idea. They offer access to the same marketplace as the official Steem Monster Market, but provide a much better UI, especially if you want to buy a lot of cards at the same time.
To use Peakmonsters, log in with your Steem account on https://peakmonsters.com/ and click on the "Market". Find the cards you want to buy, add them to your cart, and check out once you have added all the cards you wanted to buy. Checking out is done via Steemconnect as well, so there is no need to give your keys away in order to use this market, making it a safe alternative.
At the end of the day, both markets provide access to the same cards for the same price, so use the ones you like the most.