I can't find the exact thread, but I do recall a BPL member talking about making VBL sleeping clothes out of garbage bags + duct tape. Another option might be one of those super cheap rain suits from Coleman or whomever. I bet those don't breath much. Bed Bath and Beyound sells emergency ponchoes for a dollar or two and they probably don't weigh 3oz.
Maybe not great for long term, but for experimenting with VBL, a cheap option.