Best Replacment Battery for the Sega Game Gear Battery Pack

I've bought a Game Gear from Ebay that's in a case with all of the stuff for the game gear and the Battery from the Battery pack is dead from what the seller had said.

I'm trying to find a good battery to Replace the Battery Pack so I can have it on the go if I need to power it up once the Duracell rechargeable batteries die and this is my option to use.

Here's the size it needs to be - 130mm L x 45mm W x 24mm H and it needs to be at least 5,000mAh for it's capacity, I've looked on HobbyTown's site and nothing didn't found a battery to replace the one I'll get very soon and I'll have to mod the wires to have the connector of the original one that was on the battery unless there's one that can fit the Game Gear's Battery Pack.

So where I can buy the best battery to get this all fixed up?

PS - it can be Nimh type if the old classic Nickel ones are discontinued.

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