Welcome to the forum
To give a little personal input (not advice) but look what is above the area that you want to put the side pipe length at, meaning window, door. It would be my personal input not to install/terminate the side exhaust under a window or a door where one can get out of the van and get burned by the exhaust (most side pipes have covers, but it's the exhaust either coming in the window when van is at idle or someone getting out of the van getting burned by the exhaust)
Best place to look for van parts would be here or E-bay and if you have a Facebook account then you may want to look at a van group. If you want stock replacement windows, check out http://www.parkin-acc.com/index.html
Best of luck and hope you stick around this site....