As notom stated, the classifieds are pretty much your only option.
That said, something to keep in mind: if you only have enough left for the launcher then you don't have enough. The real cost is in the grenades. For a decent loadout of grenades you're looking at the cost of a higrade rifle. I bought six propane 120 rounders and they cost me nearly $500 including taxes and shipping. Of course I bought them new, you may be able to get them cheaper second-hand, the only problem with that is they're practically impossible to find second-hand.