Have a look at:
http://www.burgessae.co.uk/eshop/en-gb/dept_44.html
Electric fencing reels for farming
Mini SI Reel
£19.07
Rutland - Electric Shepherd
Holds 250m polywire or 100m 12mm tape.
3B 19-180
Hand Mounting SI Reel
£28.71
Rutland - Electric Shepher
Holds up to 750Mtrs of poly wire
3B 19-190A
Polywire is about 3mm dia.
I have a couple for cave diving ,you need to cut of a few bits to stream line them. Other farming stores stock them (Countrywide etc).
Andrew.