I am looking for pumps suitable for pool filtering that can run on DC current.

I know about one company (Lorentz) that sells these pumps but the price seems abusive if compared to an AC pump of similar capacity.

What I need is a pump that can handle a volume of about 15m3/h at 1.5 bar(loses from the filter in the pool).

Do you know where I could get such a pump? Or should I mount a DC motor myself? 

Your alternative would be to run the solar array at a high voltage and feed the DC to the DC-Link of a VFD.  It would then put out 3ph for a 3ph pump.

Um, wow, that looks to me to be about 2.5kW or 3.3HP.  Ouch.
2.5kW / 200W/panel = 13 panels for the required power at local noon.

Looks like you will need something to PPT and run the pump at the available power.  Note that centrifugal can get really inefficient at lower speeds.  Could be a positive displacement pump would be in order here.

Pressure drop on a clean sand filter is about 1 psi, backwash when you have 10 psi.  assumes a #20 grade media, 4 gpm/sqft,  (Sorry about ft-lb units)
Most of the energy requirements for filtering a pool are from the minor losses; skimmer weir, drain strainer, fittings and elbows, backwash valve, and the return fittings are also a large minor loss, as they are a nozzle to aid in mixing the pool water in the pool.  Standard losses from the pipe lenth depending on how far away the filter is from the pool will also play into this equation.
If you reduce the minor losses, you may need to filter more water per day to keep a clean pool, as the one turnover per day is based upon a good pool water collection and return mixing systems.  Also, if you lower the mixing in the pool, the use of a solar cover will drastically degrade water quality.

So if you are looking to filter the pool with solar power,  look at the whole system, reduce the losses which will reduce the pump size, in turn reduce the size of the panels.  I assume this is new and not a retrofit.  The cost of upsizing all the parts and downsizing the pump is minor compared to the cost of the panels

Keep in mind the affects on the water quality as you change the filter system design.

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