Where can I find (online) manuals for refueling A320/B737 for a given destination?
Very rarely will a manual directly be used by flight crew for calculation of fuel uplift.

Flight crews generally receive an Operational Flight Plan prepared by the operator (the airline), or an agent of the operator, where calculations have been made for how much fuel will be required for the specific flight, and necessary reserves. Flight crew may elect to revise the fuel required to provide additional margin for weather, traffic or other factors.

The calculations made for the Operational Flight Plan are made with flight planning software, which in its turn uses data from performance manuals/data sets, and takes a large amount of parameters into account (routing (duh!), wind, flight altitude, speed, aircraft weight, load distribution (c.g.), alternate airport availability etc). 

Flight crew can however make simplified fuel calculations using the FPPM for the aircraft type (Flight Performance and Planning Manual) (Boeing). The flight profile is however generally quite limited to a few climb/cruise/descent schedules, and the data is not what would be used on a normal line day. Distribution of FPPM (and other manuals) is generally restricted. If you however would come across a replica of a FPPM, you could probably make a usable fuel calculation using the instructions within it.