Re-marry

According to some sources, Grelka is supposed to be available to the player as a potential spouse after the amulet of Mara has been acquired, but a bug prevents the dialogue option from opening, possibly due to a removed or bugged quest. This can be solved on the PC by targeting her in the console and using the command "addtofaction 19809 1", as well as the code "setrelationshiprank player 4 " which will open up the option for marriage. However, this is done at the player's own risk.

With 1.3 patch, the bug still needs fixing, but only typing "setrelationshiprank player 2" on her will do the trick.

This should enable the marriage dialogue options.

Maramal can be found in Riften. He is either in the Temple of Mara or the Bee and Barb. Sometimes I see him roaming the market as well.

Here's a quote from the UESP wiki regarding Maramal:

Maramal is usually found either in the Temple or walking around the city. He is also found, occasionally, in The Bee and Barb.

...

Upon your first visit to Riften, Maramal can be located at the Bee and Barb. There is a scripted scene of him preaching that is activated upon your entrance into the inn. Until then, he will not leave and cannot be found anywhere else.

And yes, Ysolda can be married.