£6 should buy you 30 credits on the Scotlands People website
http://www.scotlandspeople.gov.uk/I searched the statuatory marriage registers there and found your grandmothers parents married in 1890 in Ballingry District, Fife. Once you have bought the credits, you can download their marriage certificate using the details I've provided above. The certificate will give you more information about them, and, hopefully, their parents names too.
And once you have the certificate, you should still have 24 credits left over to dig deeper into your ancestry!