A P90 In 40.S@W
Not sure I,ve ever seen one in the caliber. You sure you dont mean a P94. The P90 I have and all the ones I,ve ever seen have been in 45ACP. If you are really looking to get a 40S@W/9MM combo gun I would recommend getting the Beretta 92/96 kit I think you can get them for about $1500 new. But as for putting a P90 slide on a P89 I would reccomend aginst it. Also if you like the 40 S@W why not just get a gun in 40S@W such as a glock you can get them new for around $550.