hello,
we will visit Split in the second half of July.
Is it easy to find a place for 40-50 euro ( double room / apartment).
Do I have to book it in advance ? When yes, any tips where to book it.
thanks. heinoo

Arrive as early in the day as possible to maximise your options and ideally not at all on a Friday or Saturday. The area underneath Marjan Hill can be very appealing as a place to stay if you can locate such a pansion.
Also - consider Trogir as a base instead if it suits your plans.
Your budget seems realistic.

Guys Trogir is excellent base and enjoyful town you should see it!
I belive you won't have problems with your accomodation in Split because at 14th July Ultra festival is over (almost all rooms are sold our and prices are higher for that period).
I have a friend that rents an apartment in Split centar (if you still haven't find your accomodation there).
Anyway if you need transfer from Split airport you can check low budget [www.transfer-split.com] or taxi to Trogir or bus 37.
These are also options when you go to Split.
You won't make wrong choice no matter what you choose,
Cheers ;)