A cartridge with SD card support
You can purchase Everdrive at the official site for 175 USD: Purchase Everdrive 64
TW RGB - N64 RGB MOD install service form:
If you are going to purchase yourself : Purchase.
Some of the later PAL models of Nintendo 64 have a fine pitch DAC chip: MAV-NUS or AVDC-NUS. This requires an extra adapter.
If the video chip of your nintendo is one of these, make sure to request to the merchant the DAC adapter