I struggled to find a cost effective one, eventually found a neat piece of kit from a company called Cruz Pro (from New Zealand - i hadn't come across them before).
The UK distributor /stockist I used is Energy Solutions - really helpful and knowledgable. They seem to do a wide range of Boat gauges and were a lot cheaper than alternatives I looked at.............http://www.energy-solutions.co.uk/cruzpro.html
IIRC was £50 mark...I bough the one with the seperate sensor and mounted the gauge on the dash....works too, as I found out when my sender unit plate worked a bit loose on top of the tank....
Hope this helps,