Availability of each size of keg will vary by brand of beer and location. You should inquire with your beer supplier to find out which brands of draft beer are available in your area for immediate pick-up, which ones need to be special ordered, and in which size keg those particular brands are available.
When inquiring with your local draft beer supplier, keep in mind that they usually only stock the most popular brands of beer and keg sizes. But local draft beer suppliers usually get deliveries from the beer distributors or breweries once per week. If the brand or keg size you are interested in is available, and if given a few days notice, they are usually happy to add it to their next delivery.
As there are literally tens of thousands of breweries and brands of beer available, it's simply not possible for Micro Matic to maintain a listing of the sizes of keg, or area in which, each brand of draft beer is available.