I'd take a look through the forsale section, just jump on any good branded one when it turns up. I'd be happy as low as 3x at that range up to about 9x, but thats just me, I guess my scopes spend most of the time on 6x.
I've got a hw99s too, lovely little thing after I'd removed all that rubbish yellow grease and put some molly in. I've also buttond the rear of the piston, very fast and slick to shoot now. I may button the front and try to smooth out the cocking link when I'm next in a tinkering mood