You should/will be able to order a front caliper rebuild kit from your dealer containing all the rubber bits: piston seal rings, piston dust boots, guide pin dust boots and bleeder nipple boots plus a bit of high temp silicon brake grease. One kit does both front calipers. Not so cheap as the rears, so try an on-line discount dealer like Tasca. I've never seen them in the aftermarket. You'd probably have to order in bulk to get them like the re-builders do.

Running without the piston dust boots in place is not a great idea. The piston will get scored from grit and corroded from water and road salt. This will then damage the piston seal. The subsequent brake fluid weeping out past the seals will cause a build up of caked fluid and dirt that will prevent the piston from properly retracting (relaxing) when pedal pressure is released. This ultimately leads to a frozen piston, increased rotor wear and overheated/warped rotors. A damaged seal will also allow hygroscopic migration of water into the caliper brake fluid (not good). Unlike the rears, you'll need to pop the pistons right out to replace the front boots, so always replace the seal rings at the same time.

The primary problem area with single piston front calipers seems to be the caliper guide pins. If their dust boots get damaged or become unseated then moisture (and dirt) can penetrate allowing the guide pins to lose lubrication, eventually rust and bind. Additionally, as the guide pins wear through normal use they become sloppy eventually allowing the outer caliper shell to tilt enough to bind. Either of these problems will lead to a frozen caliper (again, not good). So as your calipers age you should increase attention to caliper guide pin cleaning and lubrication. Replacement caliper guide pins are available, but I've found the primary wear area is the caliper frame guide pin bushings which are not replaceable (except perhaps the Bendix ones as I recall), although a machine shop could drill them out and press in new ones if you've got the exact bushing spec. As I also like to maintain my own calipers I'm highly tempted to pursue that latter idea when I get a chance as my original 940 calipers are getting a little long in the tooth. On this board many years ago there was a discussion with a Master Tech indicating that there was a tolerance issue with those guide pin bushings in re-built calipers and caliper frames, even those purchased from Volvo. The suggestion was to take your own guide pin to see how tight a fit it was (yeah, like the parts department is going to let me do that).

In addition to caliper guide pin cleaning and lubrication, piston cleaning and working the pistons in and out can extend caliper life. Newer calipers should be inspected, cleaned and lubed with every brake bleed or every second pad change. For older calipers it should be done more often, especially if you've experienced any problems.
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Dave -940's, prev 740/240/140/120 How time flies when you're having fun