My search for T Molding was fairly simple. There are a number of places here in Australia that sell it. Most online arcade shops have some available – it all depends on the thickness, colour and texture you are after.
My first purchase was from GameDude, which had the Galaxian Green I was wanting for my cab. This was priced at $12.50 per 10 feet. I thought this was ok. But I have since discovered T-Molding.com which has an awesome selection, and is even cheaper (despite being located in the states). For the same Galaxian green, I was able to purchase 20 feet for $10US (roughly $11AU). When postage was added, I was up for $20.78AU for the 20 feet (compared with close to $30 from GameDude).
T-Molding.com is well worth checking out when you are looking for t-molding for your arcade cabinet. The only real drawbacks I found was you are forced to purchase in increments of 20 feet and maybe have an extra few days to wait.