146 MHz filters
170MHz Low pass transmit Filter
Minikits in Australia make a really good low pass filter which is ideal for use with the Portsdown or other 146 Mhz DATV transmitters.
Low pass 7 pole 144 MHz filter
A simple DIY band pass filter was originally published in the RSGB VHF manual and described in CQ-TV 250
The coils are wound on a 3/8" or 9.5mm drill.
Example of the design built by M0YDH with help from G8GTZ who formed the coupling capacitor between sections from the loop of red wire and the test results from workshop at CAT17
Mini-Circuits RBP-140 Bandpass Filter
Mini-Circuits make an excellent low power Bandpass filter - the RBP-140 which passes 130-150MHz signals and is for mounting on a PCB. Very low insertion loss. About £16 each or try your luck with an application to their EZ Samples programme.
144 MHz band pass filter by YU1LM
This is a nice easy filter to build and can be used on the output of the Portsdown transmitter before any power amplifier stages.
There is also some discussion on the Funcube dongle website of the YU1LM filter http://www.funcubedongle.com/?p=456
Manufacturers of 144 - 148 MHz filters
- Anntenae-amplifiers in Serbia do some very nice filter for all bands at reasonable cost - the 144MHz unit is very impressive: http://www.antennas-amplifiers.com/144MHz-Bandpass-Filter-BPF
- OCI is a Canadian manufacturer 
- QRP-Labs sell a 144MHz low pass filter kit for <10W duty at $4.65