Search in the App Store to see if they have digital versions.
In some cases they have web apps. These will not show up in the App Store. Instead search the publication's site for mobile, iPad, or other references. Web apps are basically specialized web pages that you can save to your Home Screen. Some are free, some are subscription, a few come free with you pulp subscription.
Newsstand is new, so some magazines them will appear in your Newsstand folder (and you can search for them there); others will show up as stand alone apps. When looking in the App Store check the category of the app. If it does not say NewsStand (not just News), then it will not show up in the NewsStand folder. Web apps won't show up there either.
Subscription options vary, a lot, so read the descriptions closely.