Mezzanine/non-SBA debt for seller financed deal

Does anyone have experience with raising likely non-SBA debt to fund a small portion of a deal that's largely seller financed? I'm looking at something that would be 70% seller financed, and ideally I'd want to use more debt for a further 15% rather than my own equity or having to raise outside equity. What does this debt usually look like in terms of structure, terms, rates, length etc?