you cannot buy a boss kit unless you have the dimensions you need for the steering wheel to fit .
boss kit's can be found on Ebay between 19-60 eur but they have different fitments ( wheel side . eg, bolt fitting, number of holes,distance between them ) . If you like that steering wheel , buy it and then compare a few boss kits that other owners have OR pm the fella who is selling it and ask him what boss would you need ( what boss kits are compatible with the wheel ) .
In general a MOMO boss kit would fit most of the steering wheels but unfortunately they are the most expensive.
Mitsumaniac was selling a boss kit some time ago
viewtopic.php?f=4&t=10292&p=80250&hilit=boss#p80250 , worth PM'ing him and have a look .
PS : personal opinion , there are nicer steering wheels out there mate