Yeah that's one thing you don't need, at least not in Buenos Aires. Maybe out in the sticks near Bahia Blanca where Steve lives they don't have it
I just had someone bring me 4 200-count boxes of ziploc bags. The Tupperware-like products sold here (I've not looked at actual Tupperware to compare quality and price, though my wife told me there's a Tupperware store on Santa Fe) are usually pretty crappy - hard to seal or don't seal well, don't last long. The ziploc bags are sold in small packs, like quantities of 5 or 10. The last time I looked, which was some time ago, the 5 count was something like 20 pesos. Cost as much as one box of 200 I just received from the States.
As mentioned, if you do a lot of cooking, bring spices. I also just had some Old Bay and chili powder delivered to me. You used to be able to find fajita, taco, burrito, guacamole (etc) mixes but I haven't seen any for almost a year now. You might be able to get chili powder in Barrio Chino, but I wouldn't depend on it and I wouldn't be sure what chilis were used to make it (unless imported).
Don't plan on buying clothes here. Bring as many as you can. You can find things here, but anything of quality is expensive, and anything not of quality doesn't last, sometimes for more than a few washes - a lot of people wash some of their clothes by hand because washing machines tear them up, even on gentle cycle. I wear X-Large shirts and find that here I have to wear XX-Large (I'm not fat, I just don't like tight-fitting clothes!). It seems that shirts are one size smaller and XX-Large can sometimes be problematic to find what you want when you see it.
Bring as many electronics as you can. Some things you'll have to buy here because of the size, most likely, but any devices that fit into luggage try to bring.
But, if you live here for awhile, you'll start to learn to do without a lot of things you're used to, or find a way to get the things that become a priority for you after being here for awhile.