I have Metal Slug Anthology, Metal Slug, and Metal Slug 2 all as downloads from PSN. They are all still available. The Anthology doesn't have the option to turn off "smoothing", but it does offer three view modes: 4:3, original pixel (which runs in a much smaller window), and full screen (stretched and distorted, but playable). The standalone games (Metal Slug and Metal Slug 2) are great arcade ports that do offer some additional tweaks, notably you can turn off "smoothing" and get a more original, pixelly look and feel to the game (which is better to me).
Unfortunately Metal Slug X on PSN is a PS1 port, NOT a port of the original arcade game, and there are no other standalone ports like Metal Slug 3, X, 4, 5, and 6 available. But there is Metal Slug XX, a much better port of the original Metal Slug 7 that came out on the DS (better graphics, bigger screen, etc.). All of these are available through PSN still, I just checked the store through my PSP. The Anthology is 19.99 (as is Metal Slug XX), and the standalones are like $7.19 each for PS+ users, 9.00 for non PS+ users.
I still own the Anthology on UMD (as well as Metal Slug XX), but wanted the downloadable Anthology just to have it easily available (and it does load a bit faster, though load times are always an issue with SNK ports).