Certain embassies and consulates(Bali-where I am headed-is one of them) require proof of outbound airtravel to issue Tourist visas. As far as I know the thai consulate in Denpasar gives double entries, if you can show 2 outbound flight bookings, which is a pain in the neck if you want to leave your future travel options open.You can book 2 cheap flights out of Thailand(the cheapest I could find around 1500 baht)and just discard them after you got your visa;but that is still 3000 baht down the drain. Or buy more expensive refundable tickets , and get a refund later, sth I have never done before:So here is my question: I assume that the refund is not 100%, so how much do you lose there?

Are there any other options?Like for example have an agency make a 'preliminary'booking?

It all depends on the ticket conditions. Basically the cheaper a ticket, the more restrictions.

With the same airline you can buy several kind of economy class tickets. The cheapest ones for instance cannot be refunded and dates cannot be changed, while the same seat with a more expensive ticket at the same company will allow you to change the date, but no refund and an even more expensive ticket allows for a refund.

Best is to check with an agent to see which ticket with which airline is best. (Might take a while to get the refund though).

You can use,. expedia.com, most flights purchase can be freely cancelled within 24h. In reality this time is extended until midnight of the following day, Pacific US time.