for an enzyme assay? your best bet is probably to do what hjackbean suggested. then clone the gene into a plasmid with some sort of purification tag (His Tag, or even better Tap tag), and then purify it careful so your protein doesn’t denature in the process.
As for getting it, there are a probably a million papers dealing with thioredoxin, so finding someone to give you some DNA won’t be such a big deal. If you prefer, you can also get it from addgene – bacterial as well as human variants of the gene are available. http://www.addgene.org/pgvec1?identifie … pl&x=0&y=0