What I want is something portable (at biggest about the size of a Wii-U tablet controller) with hardware buttons that can run some kind of emulators, at least a Game Boy and NES one. I'm also on a bit of a budget; as in the cheaper the better.

I took a look at DS flash carts since I have an old DS, but every site that sells them looks like a scam.
I looked at using a Raspberry Pi to make an emulator console, but most require a 3D printer and quite a bit of assembly.

So my question is this: can I buy/make an all-in-one emulating device for under $150?