This is how you get it:
The manufacturer's web site is at pthorticulture.com. From the front page click on Distributors and then click on Modify to select your State. There will be a few listings of distributors with phone numbers and email addresses in your state. They should be able to direct you to a retailer who handles pro-mix in small packages.|
If you are in Canada, it is much easier, just go to pthomeandgarden.com and click on Store Locator. After you enter the postal code it will show you a map with many store choices.