well I did a simple online search for tires your size and no shortage of them online, tons of folks selling them??
here is just one of dozens
See our tire deals for 27-8.50-15 Tires, Construction - Skid Steer Tires. 45 days return policy and fast shipping.
www.tires-easy.com
I cannot imagine a local shop near you cannot get them if you ask! bt then again don't know where your located ?
and
I highly doubt that 12-16.5's will fit they will be almost 6+ inches taller, and even if they fit they would be robbing a lot of power to turn them!
and pending what is inside your tires, you may need wheels as well, , I would think anyone that sells skid steer tire wheel packages on places like Ebay, craigslist, face book market place will have tire wheel combos which will most likely be your cheapest route
just remember that typically the cheaper the tire the faster they wear out!, so its not always a savings deal buying the cheaper tire!