The Carmarthenshire FHS county-wide marriage index 1813-1837 lists five grooms of this name:
David Bona married Phoebe David 11 Apr 1831 at Llanllwch; Evan Bona married Mary Arthur 7 Aug 1834 at Abergwili; Thomas Bona married Margaret Hughes 26 Apr 1826 at St.Peter's Carmarthen; William Bona married Mary Williams 7 Mar 1837 at St.Peter's Carmarthen; William Bonah [note variant spelling] married Rachel Williams 25 Sep 1813 at St.Peter's Carmarthen.
Llanllwch and Abergwili adjoin Carmarthen town, suggesting the family was concentrated in and around Carmarthen town. This geographical concentration should make searching easier. The surname is sufficiently rare in the county that you may find it worthwhile to trace all people of this name in order to clarify inter-relationships.
Of Carmarthenshire parishes, only St.Peter's Carmarthen and St.Elli, Llanelly seem to have been transcribed and published in hard copy.
Transcripts of the baptism, marriage and burial registers of St.Peters Carmarthen 1671 to 1799 were published serially in various issues of the Transactions of the West Wales Historical Society (also known as West Wales Historical Records) from vol. 7 (1917/18) to vol.14 (1929).
This journal was reproduced on CD a few years ago but I cannot now seem to find it online - the only CD that I could find: http://anguline.co.uk/wales.html
contained only 9 out of the 14 vols and omitted two of the eight that contains St.Peters transcripts. http://www.genfair.co.uk/search.php?search=west+wales&Su...
offers this CD for a very reasonable £10.00. If you bought this CD you could buy the missing St.Peter's data as xeroxes from one of the Carmarthenshire libraries - vol.10 pp.55-86 (baptisms 1699-1721), and vol.12 pp.187-224 (baptisms 1742-1764). A two-page spread fits onto an A4 sheet if it is reduced to 85% or so, this means you would have to buy around three dozen xeroxes.
This conversation from 2000 http://archiver.rootsweb.ancestry.com/th/read/DYFED/2000-03/...
mentioned the complete CD but the link to the Australian vendor's website seems now to only offer a CD of vol.4 for sale: http://www.gould.com.au/West-Wales-Historical-Records-Volume...
I can't seem to see the complete CD listed on either the Carmarthenshire FHS website or the Dyfed FHS website but it may be worth emailing them to seek further information.
Some vols are available online but they have been scanned using OCR software which risks introducing errors: try searching "West Wales Historical Records full text" - I readily found vol.9.
Many vols are currently available on www.abebooks.co.uk
for under £20 a volume and the whole set is available for £280 (= £20 a vol).
Hope this information may be of some use.