Randy, standard "economy" tuning machines can be had at just about any local music store for less than $15 (six). Once you build a couple and decide you like a certain kind, ebay and the rest of the interworld are full of options. Six individual tuners gives you more freedom in design, 3-on a side make things kind of neat too.
The options are endless, but local is an easy way to get started...
go to your local guitar shop and ask if they have any take offs; usually thekeep them in a parts drawer or cabinet when they replace sets should get them rather cheap if not free
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http://www.stewmac.com/shop/Tuners/Guitar,_slotted_peghead_tuners/E...
If you buy 3 sets, it is less than $5 per 3-string guitar even with shipping, I think.
The options are endless, but local is an easy way to get started...
http://elderly.com/accessories/items/GTM21.htm
Cheaper than that would probably mean cannibalizing a broken / hand-me-down guitar.