If you have an Android phone with an IR blaster, there is an App called AnyMote that will give you access to the LG Service Menu.What remote can I purchase in order to access the service menu? Asbl destroys some games
Other wise, search eBay for the LG Service Remote (they can be found for reasonable prices), or purchase a Harmony Remote.
I have used all three options in the past but ultimately stuck with using my Harmony Elite. The Harmony is good purchase if you have multiple devices and want to set Activities to start each device in sequence and change inputs/settings etc. It doesn't have to be the Elite or one of the more expensive Hub based ones, a normal IR one (such as the 650) will work just fine
Edited to add:- Be careful and mind what you are doing in the Service Menu. If you alter a setting that is not meant to be altered you could brick you TV.