For Marines, you have these:
http://www.denizenminiatures.co.uk/files/25mm05.htmI have a bag of 15mm Aliens miniatures, including colonists, colonists with facehuggers, and a load of Xeno Warrior figures, but I think that these have been pulled.
If you wanted to make a Face Hugger, I would guess that two Tyranid hands, glued together would give the best shape. Add in a tenticle tail and you are done - since that is essentially what they are.
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