Cost will be subjective based on your location and where you shop but these come to mind as slick round polys (I'm assume you want a round poly as its for a cross string):
Yonex Poly Tour Fire
Tecnifibre Pro Red Code Wax (now discontinued but some stock is still about)
Wilson Revolve
Grapplesnake Irukandji
Not as slippery as the above but still works well and cheap:
Yonex Poly Tour Pro
Signum Pro Poly Plasma
If none of those take your fancy, use the TWU string performance database and filter by String-to-String Friction (COF).
A tennis string comparison tool allowing the user to design reports on string stiffness, energy return, tension loss, peak impact force, impact tension increase, dwell time, deflection, etc., for user-defined impact conditions including string tension and swing speed.
twu.tennis-warehouse.com