Borates are available as dry powders (TimBor and others) or as glycol-based liquid concentrates (Bora Care and others). Glycol is intended to improve the penetration of borate into wood fibers but studies have not definitively demonstrated an advantage over simple water solutions. Glycol-based products are also somewhat more expensive than powder forms.
Both TimBor and Bora Care should be applied to dry, unfinished wood surfaces with a pump sprayer or paint roller. Two coats are sometimes needed (see product label). Powder forms like TimBor can also be applied as a dry powder to wall voids for carpenter ant treatments.
Borate wood treatments are generally not available in stores but are available here.