What I have been doing for full proxy events is printing out proxies at Fedex on their very nice color printers (total cost is about $8 for a deck). They come out looking better than just about any other printed proxies I've seen.
If I just need a few proxies then blank world champ cards are best. I commissioned a friend to do art for the power 9 since I'll use those all the time.
I acetone everything but the silver border on an UN card, and then run it through my laser printer using a high rez scan of the card. You can find them online. They are normally used for ccg emulation software. I have also used templates to make physical cards that look like the Vintage Masters cards. The art for most of this stuff can be found all over the place. Final product looks great.
Be a bit wary of world champ cards. They are not real magic cards as far as I can tell (even tore one open to check), the paper and spring is different. I think they are closer to the cheap token cards that wotc includes in packs.
If you are playing for stakes, there is a risk of being called out.
@Brass-Man I still have a stash of blank World Champ cards I got back around 2010, when they were cheap... good stuff!
That said, if you have the time, I think the results of using acetone are better. That way you can doodle your heart out to make it full art, and there's less difference between the PLAYTEST (not proxy) card and a legit card.