You can use a universal tank like one of these Canton products: Canton Racing Products Universal Coolant Expansion Tanks
The tank function is to be a continuous de-aeration tank: coolant, air, and steam bubbles go in the top hose and only pure coolant returns to the engine from the bottom of the tank. The OEM tank has many baffled chambers which allow good de-aeration in a small package. If you want to get similar de-aeration performance from a simple tank, you should get one with at least 2 quart capacity. The tank you choose must also have an upper inlet and lower outlet fitting that matches the hose sizes of the Spyder's hoses, and has a pressure cap.
I would check with junkyards or people breaking MR-2s for spares.
Alternatively, the part is still available from Toyota under number 16480-22050
Looks to cost around 100 USD in Japan, you can check with your dealer or buy from Japan through Amayama or similar services.