You can buy filters, just make sure it's the right type of filter (i.e. paper, 9mm, etc.). Most places that sell pipes will have the paper Grabow filters. For the other types you may need to go to a tobacconist or online.
You can always smoke them without the filter too. For the most part that's what I do.
I cleaned them and cut and folded sections of Pipe cleaner to fill the area. I got to get some Prince Albert we got a CW reenactment coming up the weekend before Thanksgiving.
Lots! Off the top of my head, these manufacturers offer both filter and nonfilter pipes: Stanwell, Big Ben, chacom, butz Choquin, Vauen, design Berlin, peterson, dunhill. Basically most European manufacturers make filter pipes for he European market. Brigham also makes filter pipes that use their special rock maple filter.
For the most part you shouldn't actually need the filter. Try it without one first.
I had one of the exceptions, the filter chamber had acoustic properties. I messed around for a few weeks with that pipe and it wouldn't stop whistling, with or without an adapter. I put down the tools and bought some cheap filters in bulk. I have mostly unfiltered pipes so I don't think I'll run out any time soon. Also at seven cents a piece it ended up pretty inexpensive.