I've got Ultraflex spars for Stacker, ProTeam and Super10.
The most expensive (Super10) was around £30 from Mullin Kites.
http://www.mullinkites.co.uk
I have found them really handy to have on the ProTeam and Super 10 as they seem to reduce the minimum wind speed to about 3MPH - although self lauching can be difficult so I'd make sure you take someone with you!
Less useful on the Stacker as it only appears to lower that to 5MPH when I could be flying the ProTeam anyway.
I haven't tried stacking them on the Ultraflex, as I think that may be pushing things in very light winds.
Hope that helps,
)