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Can anyone help me with this? To be specific, I would like something like the Sipahis:

I'd say check WFB, they sort of look like the High Elves to me, but you may have to do a head swap. Also I'd recommend trying to find historic mini sites. About fifteen seconds of googling led me to this one. I didn't go through any of their products, but I clicked Ancients and saw how many ranges they had and figured you could figure it out.
http://wargamesfoundry.com/

I think your best bet is to find some historical minature ranges to start out with, then you can add imperial weapons. Also, you should probably specify what tme period youre looking at, because the ottoman empire covered centuries, and the uniform can vary greatly depending on the time period you choose.
If you dont like the historical ranges, you could also try attilan rough riders. With a little conversions they could turn out pretty nice. Keep in mind it could be really expensive though.
Lastly, i believe its spelled spahis, but that could just be english translation or alternate spelling or something.

Not quite similar to what you posted as examples, but I've seen mock-ups of vostroyans with a headswp for turbans around spike helmets. They look quite like 18th century ottomans in style, and very cool I thought.