an original spoiler or scoop should come with screws and fittings, but non genuine stuff probably won't
I used to have an aftermarket boot spoiler (hollow fibreglass), it ended up getting cracks in it and filled up with water, ended up paying for a genuine one anyway (fibreglass with rigid foam inside)
are you happy to drill extra holes in your boot lid to get extra fixings???
if you are looking at blobeye sti alloys, be careful with the PCD. the 05 reg onwards has a different PCD and won't fit. all up to 05 reg were 100x5, 05 sti onwards is 114.3x5
put a pic up of the ones you like and I'm sure someone will be able to point you towards some very similar and a lot cheaper