Okay. Let's go through what you're actually looking for, because you have the model thing a little off.
MS and the toilet number plus the seat number applies to one-pieces.CST applies to toilets without the seat. Which most of the time would be the two piece, but in the one-piece, it's the number stamped in the tank, less seat. edited by Terry
SO...the Round Bowl (Original Version) Ultramax is MS853113, plus a letter suffix indicating the features of the toilet.
"S" means the 1.6gpf GMax. "E" is the 1.28gpf Watersense.
(114 is the elongated seat, 113 is the round seat).
So, like MS853113S or MS853113E. That's a toilet you can find online and at your local distributor.
[NOTE WELL: This particular toilet is still available out there with the old Power Gravity flush, and in that configuration is called the Ultimate. It is the model number without a suffix, e.g. "MS853113". Beware because the GMax or EMax flushes are just better. Since the Power Gravity was discontinued a while back, they now all come with suffixes after the model number. Don't let someone sell you an Ultimate as an Ultramax based on confusion over the model number including suffix.]
Problem is the Round doesn't come in an ADA height. The Ultramax II comes in Universal Height standard, but doesn't come in Round. The Elongated Original Ultramax comes in an ADA Height: MS854114SL.
So, if you really need Round (we recommend elongated unless you really, really need the space because otherwise a door would hit it or such), it doesn't come in ADA.
If you can do with elongated, then you have a choice between the MS854114SL (ADA) and the Ultramax II, which comes standard with Sanagloss, 1.28gpf, double-cyclone and Universal Height at a price pretty close to the ADA Ultramax. That's model number MS604114CEFG.
C=Double Cyclone, E=1.28gpf, F=Universal Height, G=Sanagloss. CEFG is the only way that model comes.
As to where to buy locally, pull out the yellow pages, look up "Plumbing Supply" and call around. Sound knowledgeable and get a quote on the exact model number and color you want. GET NUMEROUS QUOTES, because the prices will vary wildly. You can look at Terry's prices in the red box above to see what a good plumber or plumbing supply might charge; you might be able to beat that price locally or you might not be able to achieve it locally. You also might try a plumber or two for an installed price if you're not going to do it yourself, and compare that to what you could get it for plus what someone would charge to put it in.
Good luck and come back with any other questions.