In the UNWTO (World Tourism Organization) e-library you can find domestic, inbound and outbound tourism statistics via the tab "statistics".

The UN World Tourism Organization (UNWTO) e-library is a database offering UNWTO publications on tourism. Also recommended for the latest statistical information on inbound, domestic and outbound tourism presented within the “Tourism statistics” section
Abbreviations used in UNWTO statistical overviews (Series column):
* TF = tourists at frontiers
* VF = visitors at frontiers
* TCE = international tourists at all forms of commercial accommodation
* THS = international tourists at hotels and similar establishments
* n = by nationality
* r = by country of residence

Statista is a statistics portal that provides access to statistics and studies on over 80,000 topics and areas of interest. Benefits of Statista services include: Industry-specific economic macro data with key indicators of market trends Industry-specific surveys on relevant trends and forecasts Consumer behavior studies. Statista.com provides companies, business customers, research institutions, and the academic community with direct access to quantitative data on media, business, finance, politics, and a wide variety of other areas of interest or markets.
Eurostat is the Statistical Office of the European Communities. You can also search for tourism statistics.
Look for the (national) statistical office / bureau of the country of your research and see if they have publications on tourism or eg economic surveys in which a chapter on tourism can be hidden.
A statistical office website possibly also offers a statistical database to search in like Statline of the CBS (Netherlands).
For instance for the Netherlands use Statline
Look on the internet for the National Tourism Organization of a country. You can try names like National Tourism Authority, National Tourist Authority, National Tourist Board, National Tourism Office etc. Many times statistics or policy/marketing reports are accessible on these websites
Most National Tourism Organizations have a promotional consumer site and a branch site. To get this information you need to be on the branch site of the organization.
TourMIS provides free access (register to receive password) to the ECM (European Cities Marketing) and ETC (European Travel Commission) databases on bednights, arrivals and capacities in European destinations (cities and countries) and tourism in Austria. All information can be retrieved in the form of tables and graphs.
You can go to TourMIS here.
Statistics on tourism can also be published in a paper version. You can find these report in the library catalogue by searching on the country name followed bij the keywords tourism and statistics.

Searching for books, theses, traineeship reports, dvd's en games from the collection of the Library
E.g. for the Netherlands the "Trendrapport toerisme, recreatie en vrije tijd" is published every year. NBTC/NIPO publishes the Continu-vakantieonderzoek (CVO) and Continu VrijetijdsOnderzoek (CVT) every year.