Email me with your contact info and the details of your order.
Show people this QR code on your phone when you give them buttons, and have them scan it with their phone.
This is the cheapest and simplest method, but it can be annoyingly time-consuming if you're giving out a lot of buttons (e.g. at a protest or family reunion), and people aren't as likely to spend much time looking at a website they opened in such a casual way.
Print mini-flyers from this document (English) or this document (Spanish), either to distribute or just to show people when you give them a button.
This is especially useful when you can find people who will take whole bags of buttons (with one flyer apiece) to distribute to their friends and family! But it also needs a lot of paper if you're giving out flyers with single buttons.
Get custom stickers printed and stick them to the back of the buttons.
This costs some money, but usually much less than the buttons themselves. If you don't mind waiting a few weeks, the cheapest way I've found to get waterproof stickers is through a company in my area called Sky Printing, which will give you over 1,000 stickers for $54 plus tax and shipping. You can just email them a request (fill in the blank for the number of stickers), then call 206-933-5900 to give them your payment info.
Other providers will charge substantially more but get you the stickers much faster. Generally you'll want to download this image file to print 1" circluar stickers. For example, MakeStickers.com will print 1,000 waterproof circular stickers for around $100. Here are detailed instructions for MakeStickers.
Smaller stickers that fit on buttons smaller than 2.25" may be hard to find; please email me if you need help with this.
Print out QR code labels from this document (as many copies as needed - each page has 84 labels), and glue them to the back of the buttons.
This is very cheap (some public libraries will even let you print several pages for free), but also very time-consuming because you have to cut them apart and put the glue on by hand. You will need a glue that works well on metal, such as Gorilla Super Glue or E-6000.
The label document is designed for 2.25" buttons; please email me if you want a version that will work on a smaller button.