Here's another one, cut & pasted from maplin electronics website in the UK,
I have one of these and it does work.

Coaxial to Optical Digital Signal Convertor See Prices Below

Converts the electrical output from an S/PDIF coaxial output to an optical
Toslink signal. The unit is just 72 x 48 x 22mm and features a phono input
socket, a standard Toslink output and a DC power socket. Power requirements
are 6V 250mA, supplied by the plug-in mains adaptor included.
Ě Converts electrical digital signals (phono) to optical (Toslink)
Ě Includes plug-in mains power supply
Ě Solves problems connecting digital audio equipment with incompatible
Ě Ideal for Home Cinema and hi-fi use

Prices: (Spend ú30 and carriage is FREE to the UK mainland)

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>From: "Reckase, Erik Nathan" <enrecka (AT) sandia (DOT) gov>
>Reply-To: SlimDevices Discussion <discuss (AT) lists (DOT)>
>To: "'SlimDevices Discussion'" <discuss (AT) lists (DOT)>
>Subject: [slim] OT: Coax to fiber convertor
>Date: Tue, 9 Dec 2003 07:56:20 -0700
>Search for 'coax toslink' - there's a box by M-Audio that can be
>used not only as a converter, but a repeater as well. $44. If I find
>anything else, I'll let you know.
>-----Original Message-----
>From: Matthew Fusfield [mailto:mfusfield (AT) ins-business (DOT) com]
>Sent: Tuesday, December 09, 2003 7:33 AM
>To: discuss (AT) lists (DOT)
>Subject: [slim] OT: Coax to fiber convertor
>Sorry for the slightly OT post here, but I'm hoping someone here will have
>an idea:
>My reciever has a number of digital audio inputs, but all are fiber TOSlink
>jacks. I recently added a device to my setup that only has coax out for
>digital audio. Anyone know of a converter that will take the coax singal
>convert to fiber? (I wish everything was as nice as squeezebox about
>including both kinds of jacks!)
>Matthew I. Fusfield
>Director of Data Products
>InterActive Network Systems, Inc.
> <>
>(856) 401-9000 Ext: 1015