94 Mazda Miata Coolant Problems

Hi there. I have a 94 Mazda Miata with 109k miles. It's 4 cylinder, manual. I've had coolant problems for almost 2 years now. However, I haven't really driven my car a lot in the last 2 years. Probably, a total of less than 9k miles. But now I'm driving it a lot more and it's summer, so it's definitely become a huge problem. My coolant and the water in the reservoir leak/disappear after driving only a couple days. There's nothing on the pavement under the car, so it's not just leaking. Also, after I've driven and turn the car off, I can hear like a hissing/gurgling-like sound in the dashboard. So it's overheated many times, and recently, I've just been filling the coolant every couple days so I can keep driving. Back in December, it overheated and I didn't have any coolant with me, so I had it towed to a Belle Tire, and they said it just needed a new heater hose. So they replaced it, but I'm still having the same problem. I've been told by random guys that it's either the heater core or gasket. But there's no liquid leaking into the interior and my heater works fine, but the air is moist in my car, but I have cracks in my soft top where rain leaks in so the moisture could just be from that. Also, I haven't seen any smoke from my exhaust and my oil is fine.

-Do you have any idea what it might be?
-Do you have any idea how much it would cost to fix?
-Do you know of any good mechanics in the Detroit area? (The dealership screwed me over big time a couple years ago, so I refuse to go back.)

Thanks!