I just did a quick search for a grille for a 1959 Bonneville and there is a salvage yard in New Mexico that has one. Search for other parts and you might find more.
Good luck. It'* good to see a good old car being restored -my dad is restoring a 1956 Olds 98 Holiday 4-dr Hardtop right now and also is putting the finishing touches on his Hemi-powered (331 CID) 1953 Chrysler New Yorker Deluxe Newport 2-dr hdtp.
Here'* the info on the grille (appears they only have a right side).
Model Description Condition Stock# Price Dealer Info
Pontiac -BONNE,GOOD,W/PL,R/* IDE DT67 $40 Coronado Auto Recyclers, Inc. USA-NM(Albuquerque) E-mail 1-800-482-6394