+13 dBm reference is fine for the SA44B (it will tolerate up to about +20 dBm). The circuitry prefers square for lower phase noise, but +13 dBm has a pretty fast slew rate, so it probably doesn’t matter much.
If frequency doesn’t have to be exact (+/- 0.1 ppb typical), you might be able to find a used LPRO Rubidium standard for $150 or so.