l brought a used boat with twin stern drive engine 250hp, where can l buy cheap and good engine for my boat, used or new is ok.

These r GM manufactured engines that r ''modified'' by several Marine Manufacturers. They Make the exhaust & other castings. Keep in mind that several of the other parts, look like automotive parts, but have been modified for safety.
There r lots of them in junkyards, & available at ''engine re-builders'' but be careful, there r some that have right & left rotation etc. Some have electric fuel pumps, others mechanical. But, the blocks & heads r available locally