Can i but a sticker pass at the entrance?

Yes you can buy a daily pass for $10 or an annual one for, (I think) $40.

Do veterans get free or discounted pass

Yes you can

Yes

If you know a senior your pass is even cheaper I think it's $24 not quite sure.

Yes it 35 dollars for the year to all metro parks or 10 for a day

You can buy a sticker pass at the entrance when a gate attendant is there

Yes, you can buy an annual pass at entrance.