Hi John,
25% of Dutch homes have a caravan - you'll have no trouble finding stuff in shops, but if you need anything for your caravan, try "Camping Sport De-Wit" in Schijndel near Den Bosch ('s-Hertogenbosch) - 70,000 sq. metres of camping and caravan stuff - just amazing - and the prices (should) be lower than the UK.
There is no difference between the Butane (or Propane) gas supplied in the UK, Holland or anywhere else in Europe. There are physical differences in the bottles and are not interchangeable.
So take a full UK bottle (or bottles) with you and buy a Camping Gaz 907 (in Holland - cheaper) if you need it. Camping Gaz is universally available, but the running costs are twice that of the larger bottles.
Robert