Mass market frames will usually be found in photo shops, home furnishing/lifestyle shops, stationery shops. Even Daiso should have some cheap 4R (3:2) frames as it's what is commonly printed.
Just took a look at the ikea site, quite a number of frames for 3:2?
E.g. this looks like 3:2. (30 x 21cm)
http://www.ikea.com/sg/en/catalog/products/70301448/
Actually, all the RIBBA frames look close to 3:2 with the mat, 4:3 without -
http://www.ikea.com/sg/en/catalog/products/50301618/#/50191865
This one is 3:2 without mat (15 x 10cm) and with mat (12 x 8cm)
http://www.ikea.com/sg/en/catalog/products/70167609/
Maybe you want to look again.
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What's your use case? Is it meant to be a cheap/budget kinda thing in huge qty (e.g. corporate gift or something, home wall display, etc), or are you talking about framing one special pic?
If you have a very specific thing in mind, then custom is the way to go.
If don't have the budget for custom frames, and it's for like personal home use, I'd go and find the frames I want to use first, then crop/print my pics to fit. Not the other way round if budget is your priority. The other option if you don't want to compromise on your aspect ratio is to learn to custom cut your own mats. Then you can use almost any off the shelf frame you want. Got some sunk cost involved, buying the mat cutter + practice, but if you need it consistently, then it's a worthwhile skill to have.