Latvia's Map stamps of 1918 are well-known, well-studied and widely collected - quite often, in complete sheets. There is a very good website devoted to them at www.apsit.com. I have used Bill Apsit's site for some of the information below.
They were available at post offices but for a very short time period - and most of the people at the post office counters appear to have been stamp dealers or collectors.
Here is a brief Chronology. After Germany signed the Armistice with the Allied Powers on 11 11 18, Latvia quickly proceeded to declare its Independence on 18 11 18 - though German troops and administrators remained in Latvia until late in December. The Latvian government in Riga took delivery from the Riga printer of the first instalment of Map stamps on 17 December. The first illustration below shows a blank philatelic cover cancelled 18 12 18 but this is unusual - most Riga cancellations on Map stamps are dated for the last five or six days of December. Then the trouble begins.
The Latvian Government evacuated from Riga on 2 January and on the 3 January, Soviet Latvian troops entered the city. End of Map stamps in Riga. The government evacuated first to Jelgawa (Mitau / Mitava) and then to Liepaja (Libau, Libava).It returned to Riga on 22 May 1919 and took delivery of more Map stamps (which may simply have been kept in store during the Soviet occupation). But it seems doubtful that these new supplies were issued.
So if you are looking for postally used Map stamps, then for Riga they will only be found in a 14 day period from 18 December to 1 or 2 January. After that, they can be found from other cities and towns - but rarely - and they were soon replaced by further issues with a wider range of values - the Map stamp only exists in one 5 kopeck denomination.
Philatelic productions can be as basic as these two, one prepared in Riga 18 12 18 and the other with an undated provisional cancel of Jelgawa: