yes, you have to work with a Realtor licensed in the state of Maryland and who has access to that County.
No it is not unethical, but you should make your VA Realtor aware of this and make sure that he/she understands that you are also looking in Maryland.
Your current Realtor should be able to help connect you with a good Realtor in Maryland and they then could work as a "team" to help you locate a property.
The thing is to remember that people need to be paid for their time. That is how Realtor's make their income. So you want to make sure that whichever location you buy in that the 2 Realtors you work with get paid for their time. Often times they can split the commission or sometimes do as referals. There are ways to make it work. But be upfront and honest with both Realtors.