Amazon sell jewellery and will have a range of prices, so I would try there or eBay outlets. You don't have to buy anything labelled as a promise ring, just one you like and that you give to him for it to have that meaning. Just make sure if you order online that you know his ring size, as it is not like in the shop where you could go back the next day and exchange it for the right size if it does not fit.
any ring can be a promise ring. in fact, a ring that fits his stile would be a better choice. if he truly likes the ring he'll actually were it. so don't limit yourself, its about the promise attached to it, and not the ring itself.
you can check appealingweddingbands. this a great site to choose a meaningfull ring.. i tried this site and be pleasured..
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etsy . com/searchresults.php?searchquery=promise+ring&search_type=handmade
'style' 'wear' 'it's' ...
Sorry, could not resist...
cheappromisering.com ???
lol !!