Hey,
Would you be able to tell us where you are located, as it may help us to help you?
The best place would most likely be a land-scaping supply center (I used Central Land-scaping supplys here in west Auckland). Ask tham for river sand and they should be able to help you out. But you will have to wash it very, very well!!!
I have used white river sand in one of my catfish tanks before, without any problems. I mixed the sand 50/50 with some dark gravle, and it gave quite a natural effect.
I take it you are not trying to make a full biotope tank, rather a mainly pleco tank with a blackwater river setting? It should come out very nice.
Good luck with it, and keep us posted.........
Cheers, CatBrat.