PulkoMandy is the author of the Albireo expansion card for Amstrad CPC which lets you use mass storage (USB and/or SD card) and a serial port.

With the agreement of PulkoMandy, Zik is going to make a new batch of cards (beware without the serial port). It's even possible to use two cards (but needds a cutter and some soldering).

You will need a ROM board and one of the ROM which can manage an Albireo, Unidos for example. The estimated price for the card, shipping cost and maybe a SD card is 30 euros.

If you are interested, go write on the CPCWiki's thread.