The only legal way to obtain a ti83plus.rom file is to create it from a TI-83 Plus calculator that you own. This process is known as "dumping" the ROM. You can check your calculator's ROM version by pressing [2nd] then [+] (MEM), then selecting [1] .
For more information on Ti83plus.rom and related topics, check out the following resources: Ti83plus.rom
Ensure the file extension is strictly .rom and not mislabeled as .83p (which is a calculator program file, not a system image). The only legal way to obtain a ti83plus