Welcome to the forum, Jan!
With respect to getting an inexpensive T60/X1400 (non-widescreen) motherboard, I suggest you consider one of the European eBay sites, such as e.g.
eBay.de or the
German ThinkPads forum the reason being that: 1) Inter-Europe shipping is most likely less expensive compared to having stuff sent from USA or China and: 2) You will not have to pay any import-tax (duty) if shopping inside the EU. On eBay.de you will find boards such as e.g.
this or
this ... this should give you an idea of the expected (maximum) cost. On the German ThinkPads forum, see e.g.
this T60/X1400 motherboard selling for just EU 40,- (does not seem to have been sold?).
As to what specific motherboard FRU you can use, there are quite a few functionally identical types although with different part-numbers/FRU numbers (newer versions), but unfortunately Lenovo has
not been friendly and put together a publicly-available list of all the compatible numbers. So, your best option is probably to see e.g.
System service parts - ThinkPad T60/p (non-widescreen models) and in addition to this then search e.g. on this forum - use Google to search for e.g.
"T60 motherboard X1400 FRU site:forum.thinkpads.com" for more information about the boards you come across.
PS: Be aware that you will need at least a
Revision 3 motherboard to support your T5600 Core 2 Duo CPU... see e.g. the thread
T60 Core Duo --> Core 2 Duo or
Core 2 Duo Upgrade
PS2: Strong advice: Be kind to yourself and get a SSD now that you are working on your T60... a SSD will boost new life in your ThinkPad; try see e.g. the threads
Latest Recommendations for SSD replacement to old 60gb HD or
Adding an SSD to a T60
Johan