There are two simple answers to the question “How do I buy stuff in Trimarch?”

1. Take a trip: get your sack of cash and your trusty steed and ride to Critwall, Admundfort or Radigast City and shop to your heart's content. It'll take time, but you'll find what you want.
2. Place an order: if you go to Fudley and meet with Marcello, you can place an order and he'll get what you need on his next trip to the city. It will take 1d4+1 weeks for the order to arrive, and the payment is upfront with a 20% service charge.

If those options are unattractive, it is possible to find certain things to buy in the region.

1. Food and Drink: most ordinary food and drink items can be purchased at Good Time Charlie's Trading Post, Gerk, Bromley, House Rufo and Fudley at normal prices.
2. Cheap, non-military gear: things that cost 20gp or less and are not specifically military gear can be commonly found in the same places that you can find food and drink, for example, lamp oil but not military oil.
3. Basic weaponry: spears, knives, staffs, hand axes and arrows are available in the same locations, but only in small numbers (no more than 1d6 at a time, recheck next month.)
4. Common Mounts: at the West Fudley market, you can usually find up to a half dozen each oxen, mules, donkeys, draft horses, light riding horses and ponys for sale. Of course, pack llamas can be found at Rufo.
5. Herbals: anything except Black and White Lotus is readily available in Gerk or from Sinric the Wise in Bromley.
6. Military Gear: Baron Zander of Bromley will sometimes sell a piece or two of the arms and armor he has lying around to people he likes (the right people of course).
7. Awesome Pies and Owlbear Punch are available in Bromley

Maybe, Just Maybe they'll have what you want at Good Time Charlie's Trading Post there's always a chance.