I just leave home with empty space in my luggage so that I can permit myself to buy a few dictionaries along the way. Most of my wardrobe when travelling is washable, and I bring along the most wornout rags I've got so that I can throw them out if I need more space in my bag.
The big problem nowadays is that paper dictionaries are dying out and that you only find grammars for the languages the other customers also might want to buy, and then the temptations become less numerous. But dictionaries and grammars for Kadazan and Dusun (and Murut - how can you live without it?) would definitely be worth owning.
Right now I'm in the middle of a major multilingual wordlist project. In the last round I included some 28 languages, and since then I have acquired dictionaries in Czech and Ukranian and they both deserve to be included in the current round. so far I have reached the Iberian pensinsula with Portuguese (after English, Scots, Afrikaans, Dutch, Low and High German, Icelandic, Nowegian, Swedish, Latin, Modern and Ancient French), and today I expect to do the rest of the Romance languages and if possible also Greek and Albanian and Esperanto and Indonesian. Then I'll go visit some zoos and aquariums and museums, and after that I have the Slavic languages waiting for me. Actually I could extend the project to include Hungarian, and it ought to be possible to by an Estonian dictionary over the internet, and ... well, I don't study the last handful of languages yet, but it was fun to do a wordlist in Finnish so I'm sure it also would be interesting to study the vocabulary of for instance Latvian or Lithuanian. I only leave out the Asian languages because there is such a thing as lingvistic constipation.