You can buy direct from Streamlined, they will post anywhere. Go SurfSail first though, shoot them a message, keep it local.
streamlined.us/Lot of people don't like that style. Never had an issue myself. Maybe more fiddly to attached ? but if they are it is like 3 seconds difference per rig up, zero per rig down. I'd say you can't beat Streamlined booms and bases for simplicity and strength. No fancy pointless stuff that corrodes, snaps, needs fancy washers to fit etc.
I did replace the head on my boom a few months ago, the last Streamlined one only lasted 10 years and the rubber cushions broke down. You can get new rubber bits from Streamlined but they seem pretty costly for a bit of rubber stuff, so I figured I'd treat myself to a whole new head. They do make different sizes for different boom diameters, but the new ones are all blue, my old one was red. I heard red is faster,
Oh yeah, also I only use RDM so I don't know how they'd go using the SDM version with a mast shim. Might be best to be RDM / SDM specific, dunno.