I may be a dummy but I don't know what a "Dommie" is. If you are looking for tubeless 17s like on my Cagiva Gran Canyon, you may want to ship some ahead. But I am assuming you are looking for common tube-type 17" rear dual sport tires as found on KLRs, etc. These are not so easy to find as the 18"ers but there is some availability.
Belize: Rod Allen Hardware on the Western Hwy in Central Farm (a little ways east of San Ignacio). The one time that he didn't have them, he sent me to the Texaco station in Spanish Lookout, who had them. This Belize info is about eight years old, the rest of the info here is as of 2002.
Guatemala: This is dirtbike/dual sport heaven (even though they've been paving some of the jeep trails, like my old favorite from Sayache through Chisec into Coban). In Guatemala City there are a bunch of shops in Zona 4 where I have gotten 17" tires. Most notable is VRC on 7a Avenida 8-44 Zona 4. They saved my butt in '02 because they had an Acerbis headlight that I needed after mine got smashed by a rock kicked up by a truck. They have several other shops around the country, check out http://www.villeda.com
It would not surprise me to find 17" tires in some of the other larger Guatemalan cities like Zacapa, Chiquimula, Esquintla, etc., there is a lot of dual sport bike parts availability throughout the country.
Honduras: In Tegucigalpa/Comayaguela (my favorite Central American city, with the most gorgeous girls)the Honda dealer was just setting up in '02 and didn't have much inventory. But there is a small motorcycle parts shop in Comayaguela with a medium stock of tires, I bought an 18" there for my XR 650R. I forgot the name of the street, but it is the only main street that runs diagonal to the street grid, it runs southwest and uphill off of 6a Avenida, there is an Isuzu dealership at the bottom of the hill. The motorcycle parts shop is part way up the hill on the right on this major commercial street. A little further up the hill on the right is a family run car/truck tire shop that can change motorcycle tires (cheap).
El Salvador: This country has more critical needs than motorcycle tires. Availability of food that is more nutritious than papusas would be a start.
Nicaragua: The Yamaha dealer in Managua has a very weak supply of tires at high prices and I didn't find motorcycle tires anywhere else in the country.
Costa Rica: Aside from San Jose (where I would expect good tire availability), there is a Honda dealer on CA1 a ways north of San Jose called J. Esparza. I assume he distributes throughout the country because he has a large stock of tires. He is very helpful and even changed my tire and did other repairs on a Sunday for very little money. Tel 636-7896 or cell 382-0911.
In Perez Zeledon there is a motorcycle parts shop with tires and a full line of BelRay oils called Multi Repuesto tel 771-1210. The owner is friendly and also has cabins for rent outside of town.
Panama - The only possibility would be the Panama City area but I would not count on it. Some of the Mobil gas stations carry Mobil 1 oil, though.
I hope this info is helpful to anyone riding down who wants to get an idea of parts availability. This is definitely dual sport territory - pack light!