Steve, it seems only the 250ml is listed over here - that might be why all the suppliers that list the small bottles are in the US. There does, as the guys have said above, seem to be plenty of choice for US suppliers though.
How much do you think you might use - and are you likely to need it in the future? Just thinking if you have to pay £20 for a small bottle plus maybe the £20 carriage from the states that some suppliers want, you're over half way to buying the larger bottle anyway. I don't think Amazon will ship to here free from the US.
Another alternative might be to find it surplus (though beware long out of date stock) - the NSN is apparently
8040 99 225 4168 ' Adhesive, Cylindrical Retaining'