Purchase orders can be searched for in the search bar located in the top right hand corner of the header. In addition a filter has also been added to the purchase orders page to filter the results by entities & dates.
Making purchase orders similar to quotes sounds like a nice idea. However it is unlikely that we will be able to implement it until the idea gains more support from fellow member on the community. I would recommend making a new ideas post about this, and once enough people have shown their support for the idea, we can have a look at possible implementing it.