A timebomb is basically a purposely set date in the kernel for the cart that deactivates the usability of the kernel in order to force you to buy a new cart. Retro's files provide a homebrew kernel called "YSMenu" that can bypass this timebomb and allow you to continue to use the cart past the time the "expiration date."
What I can recommend really depends on what other systems you have laying around. Every single cart on the market made by the R4i-SDHC team can be flashed to run NTRBoot, but they all have a timebomb in the kernel. Regardless my second suggestion would be the
R4i-B9S, since it comes pre-flashed with NTRBoot and can be flashed back to a normal R4i-SDHC. Or saving a couple of bucks and buying just a normal
R4i-SDHC and flashing that with NTRBoot. There are other methods, but some like the Ak2i or DSTT require additional hardware to flash NTRBoot onto.
Actually there are some 1:1 clones still being made that function the same and still work with NTRBoot. The only issue is that they don't work on anything above 4.3, so they require either another hacked system or a DS(lite/phat) or DSi 1.44/below.