What clone are you doing? If it's a trapist beer, White Labs sells the same strain that is used at the trapist monetary.
There are 7 different trappist monestaries, and at least that many trappist yeasts.
* Westmalle monastery produces yeast used by Westmalle themselves, Westvleteran, and Achel. This is Wyeast 3787/White labs 530.
* Chimay has their own yeast strain. This is Wyeast 1214/WLP 500
* Rochefort has their own yeast strain. This is Wyeast 1762/WLP 540.
* Orval has several yeast strains. One of these is WLP 510 but they also use a Brett strain and reputedly several other variations. Some people report good results pitching 510 originally and bottle-cultured yeast (which is presumably a fuller blend) or Brett at secondary.
* Koningshoeven uses at least one and possibly more strains of their own. They are the only Trappist brewery outside of Belgium (located in the Netherlands).
In addition, the St Bernardus brewery did all the brewing for Westvleteran until 1992, when Westvleteran moved production. The St Bernardus yeast is apparently the traditional Westvleteran yeast, so that's a historically trappist yeast. Westvleteran outsourced their yeast production to Westmalle (as noted above) and no longer uses the historic strain.
EDIT: Orval is 510, not 500.