Hi Arturo. 'Ostara' is very different from A. 'Purple Sensation'.
'Ostara' is a shorter Allium, with broad leaves that remain fresh throughout bloom. It also blooms much longer and the blooms are more "red" purple than the "blue" purple of 'Purple Sensation'.
I actually just posted a bit of info on the iris forum about Ostara, so I will copy it here, for you:
Allium 'Ostara' - A cross between A. karataviense and A. atropurpureum.
This is a fairly low Allium that is supposed to grow 16" (40 cm) tall, but actually in my garden it is lower. The height and bloom time in my garden is excellent for mixing with many IB's. Bloom time in other gardens may differ. The broad foliage will remain attractive throughout bloom. I guess this is a trait inherited from A. karataviense.
After bloom it will however still of course go completely dormant.
This Allium is much easier to grow than karataviense. It also has a lot nicer color for my taste as karataviense is whitish gray. For this reason it has replaced karataviense in my garden.
A word of warning though, is that 'Ostara' spreads very quickly through masses of tiny rice grain sized bulbs.
If it grows well in your area, i believe Ostara will be a good choice for a lower growing Allium. If you want big and tall Alliums with giant flower heads the very best I tried is 'Globemaster'. If you haven't already tried it, I would truly recommend doing so. If I could only grow one Allium, this would be it. It also usually blooms 3 times longer than 'Purple Sensation' in my garden as it usually blooms for 3 weeks. The flowers are sterile and the flower head just keeps on expanding during bloom.
I'm sorry, but I don't know about sources for bulbs in your area.
