Their marriage could have been either of these, searching for any marriage of John Stanbury or Stainbury with any Grace from 1790 to 1810 anywhere in Devon:

First name(s) John
Last name Stanbury
Marriage year 1791
Marriage date 22 Jun 1791
Parish St Gilesinthewood
Place St Giles in the Wood
Spouse's first name(s) Grace
Spouse's last name Cock
Denomination Anglican
County Devon
Archive South West Heritage Trust
Record set Devon Marriages
Category Birth, Marriage, Death & Parish Records
Subcategory Marriages & divorces
Collections from Great Britain
Devon Family History Society

The image adds that he was a labourer and the witnesses were Richard Smith(?) and A: Clenience Clerke.

or

First name(s)           John
Last name               Stanbury
Marriage year           1793
Marriage date           25 Sep 1793
Parish                  South Tawton
Place                   South Tawton
Spouse's first name(s)  Grace
Spouse's last name      Lang
Denomination            Anglican
County                  Devon
Archive                 South West Heritage Trust
Record set              Devon Marriages
Category                Birth, Marriage, Death & Parish Records
Subcategory             Marriages & divorces
Collections from        Great Britain
                        Devon Family History Society

The image adds that John’s parish was Hatherleigh West (or is the word ‘Wool…’) and the witnesses were John Stanbury (his father?) and Christopher Forggs.
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It was the groom’s parish that led to identifying the marriage.  He was of Hatherliegh.  Their son Henry, the child of John and Grace was born in Hatherleigh.  So his mother was Grace Long.