As of a few days ago, if your Steam account was localised in Argentina and hence in AR$, you could buy games and software with the usual Visa/Mastercard options, but also through Rapipago and at least half a dozen other local options. (see image above)
However, since the dollar fiasco on the Monday following the primary elections, these options have now been withdrawn, presumably because Steam is going to lose money.
It's also worth noting that digital purchases in Argentina, regardless of payment options, are subject to 21% VAT (IVA).
To get around these two obstacles, many are now using prepaid cards such as Ualá, Mercadopago and others. The trick is to use the prepaid card to top up your Steam Wallet where you'll get charged 21% VAT, but then (apparently) when you use your Steam wallet to buy on Steam, the VAT will not be applied.
I've yet to try this, but will give it a go when I see something cheap that's worth buying.
Mind you, it's also worth considering that for unknown reasons, the AR$ prices on Steam are considerably cheaper than their dollar prices.
For example, a game priced at US$40 on Steam's main site, is priced at AR$999 when localised in Argentina which, when converted at say AR$55/US$1, works out at US$18.