Bianca writes, "I am a citizen of Honduras and have a multiple entry visitors visa to the USA. I am currently in the US on vacation. Only after I arrived, my US citizen sister has invited me to drive to Montreal with her family for a few days, but I do not have a temporary resident visitor visa to Canada. Can I get a visa here in USA? It will be terrible for me to miss this trip as we are going to celebrate her husband's birthday."
Anyone who has been legally admitted to the United States (as you are) and is still in lawful status is eligible to apply for Canadian visa. The only difference is that the Canadian authorities might do more critical examination of your application to make sure that you are not up to something fishy and not trying to bypass the more convoluted process prevalent in Latin American countries because most countries typically like to issue visas in the native countries of the applicants (the exceptions are made when you live in a foreign country on long term visas, like student or work visas). The broad idea is that when you planned your trip to the USA as a visitor that is where you intended to visit but if you can prove strong ties to your home country, if you have a perfect record of entering/leaving the US, and do not raise any other suspicions, you should be approved.
So you can apply the same way anyone else in America will apply on the website of Canadian Immigration Services and be prepared to upload scanned copies of all the documents and pay the fees with a credit card -- in the US the process is a bit different for the application. Explain in the application that the program to visit Canada materialized after you had already left Honduras; you can provide details on the road trip and hotels based on your sister's schedule. Most likely you will be approved.