I also use human nail clippers. any sort of scissors are too dangerous so close to the piggy toes, especially if they are squirming about!
I personally wouldn't take my pigs to the vets for their nails cutting because I think they'd find it even more traumatic than me and my partner cutting them. Also, I took my old hamster for his nails cutting (he didn't move around as much when he was getting older and his nails started to curl) I couldn't physically have done it myself so I had to take him - they charged me £10 for about 2 minutes work!
I cut my pigs nails about every 6-8 weeks so it'd soon add up.
There's a guinea pig rescue near us in Leeds who have dozens of pigs and offer to cut other people's piggie's nails for a donation to the rescue, could be cash or bedding, veg, toys etc. You could look in your area?
I think the best thing to do it just get used to doing it. They don't like it but its not painful for them and they soon forget when bribed afterwards with some cucumber! its best to have one person holding the pig to their chest facing outwards and then another person to quickly hold on to the feet and snip!