Linda, the closest cemetery is Greenwood which is non-denominational, they have a burial search online you can do at
http://www.green-wood.com/index.php/GWC/39/143/detailIf they were catholic, you can try Holy Cross Cemetery in the Flatbush section of Brooklyn:
3620 Tilden Ave
Brooklyn, NY 11203-4032
(718) 284-4520
Their staff will help you if you know their death/burial dates, just tell them you are "trying to locate a family plot". With the cemetery searching experience I've had, if they pick up that you're a genealogist, they'll just tell you that burial index cards are available for $40 and have to be sent for in the mail, etc. If you get a hit there, I'll go see if there's a marker for you.
The next large catholic cemetery is Calvary in the Long Island City section of Queens.
If both of those dont turn up anything, you can try getting their death certificates from the NY State Dept of Health since the Municipal Archives doesn't have anything on file post-1930. Let me know if you need any help with phone numbers/addresses.
I checked ancestry.com for Robert McCreadie, found his Naturalization and WWII draft registration, do you have this?
It says born Feb. 24, 1883 in Cardiff, Wales. Naturalized in 1935 while living at 539 Pacific St., Brooklyn. Probably came to N.Y. about 1930 as some time passed between filing a naturalization petition, declaration of intent, and actually being naturalized. They also have a few family trees already on file for him with parents William and Elizabeth.