You can DIY it if you have some experience with repeaters. Even if you do not, find a knowledgable ham radio operator to help you. For about $500 or so you can buy a repeater, often used, which will get the job done.
Andrew | N4AWL
N4AWL | November Four Alpha Whiskey Lima | Amateur Extra
There's more to this than just finding a "knowledgeable ham radio operator".
You'll need to procure an input frequency for your system. That will likely require frequency coordination and applying for the proper licenses. This is best done by someone (or a shop) knowledgeable in how to do this. It isn't difficult, but the frequency coordination process and the necessary FCC forms are usually out of the grasp of most amateurs.
Once you have all that, you can then look into a repeater. You shouldn't need much. A simple analog 5 watt repeater in a good location with decent antennas should cover your building if you handhelds are just on the fringe. You'll need the right equipment and it needs to be properly set up. Getting the duplexers properly tuned is key to the system working correctly. With the right tools, it's an easy task. Someone trying to do it without the right tools or without the right experience is going to create a lot of headaches for everyone involved.
Your radios will need to be programmed with the new repeater set up. Make sure you keep a 'talk around' channel (or set up one of the buttons) to cover things if the repeater stops working.
I think you'll have a difficult time finding all this for $500, even if you go with used equipment.
There are easier/better ways to do this, but since you already have VHF radios, you're stuck.
Icom has a new WiFi two way radio platform that would have been a good option.
UHF would probably have worked better than VHF,
But that horse is out of the barn, too late now.