I like to use Buhner-recommended herb sources and bought my Sida Acuta leaves from Monteagle Herbs in Canada (the exchange rate is very much in our favor)
monteagleherbs.com/?route=product/listThe site has changed since I last bought from them (January). Back then, I emailed them to let them know what I wanted to buy. I'd then receive an email from Peter confirming price and availability. If all was acceptable to me, I'd email back with my intention to order and call with my credit card information.
Shipment was quick. The leaves were in good shape and fragrant (a sign that I'm sure you recognize as freshness).