I live in the US and traveling to Australia in about 3 weeks (for 5
months, so I need a long-term solution). Everything I am bringing
accepts 240/220V and 60Hz/50Hz, except my cell phone charger (which,
ironically, converts the power to DC anyway). Can I just get a
step-down voltage converter (or transformer, I don't know the
difference) and a plug adapter, or do I need to look for something
else? If so, what websites sell the correct products?

You require a 220>110 step-down transformer. A "converter" is a diode
used for converting heating appliances, and is unsuitable for
electronic devices.

Dick Smith electronics is a big Australian retailer and appears to
stock products you need:

Plug adaptor
http://www.dse.com.au/cgi-bin/dse.storefront/42ac9eb40d29311e273fc0a87f9c071c/Product/View/M7323

25W transformer- this shoul be suitable for a cellphone charger,
shaver or toothbrush, but not much else.
http://www.dse.com.au/cgi-bin/dse.storefront/42ac9eb40d29311e273fc0a87f9c071c/Product/View/M1155

65W transformer for small appliances
http://www.dse.com.au/cgi-bin/dse.storefront/42ac9eb40d29311e273fc0a87f9c071c/Product/View/M1151

If the above links time-out then go to
http://www.dse.com.au/cgi-bin/dse.storefront
click on Browse Product Categories in the l/h yellow bar, then on
Travel Accessories in the bottom r/h green block.

You might find it cheaper to buy a new charger for the cellphone when
you get there.

Owain