Most retailers have now dumped all of their 'old stock', the slim has been around for a while now. The chances of finding a JTAG'able console from a retail store is slim to none.
Your best bet for a Jasper is to see if any collectors have a sealed RE5 consoles, as they were all JTAG'able.
For secondhand consoles, you can forget all Pro's and Elite's. Then you need an Arcade which hasn't been connected to Live since June 2009 August 11th 2009 as Fast says, and not had any Wave 4,5,6 games (ie any released from around October 2009). Those arcades are very hard to come by, around 1 in every 300 on eBay UK.
The chances of finding a used JTAG'able Falcon seem to be higher. Most used JTAG's are Xenon's as people buy RROD consoles cheap and then fix them and make a quite a bit of profit. The Zephyr I sold for £125, and it cost me £100. The person who brought it from me though has a website where he's charging £250 for the console I just sent him...
In the UK there as been a massive flood of Arcade consoles given away free (such as with TV's, Mobile phone contracts). Typically they come with Forza 3 & Halo Recon. None of those consoles are JTAG'able. eBay is full of those consoles.
Also remember the JTAG'ed Jasper Arcade 256MB/ 512MB consoles can suffer from NAND corruption if the internal memory is used, so you need to have a SPI flasher connected permanantly. Which means cutting holes in the case.
One big problem i've come across, i've notice most will update the dashboard to the lastest version before selling it. They think that's a good thing to do...