I was wondering, since I want to start playing with t-budding what store bought fruit would be the best choice to toy with for practice?

Lemon Libson/Eureka?
Grapfruit any?
Mandarins?
Tangelo?

I ruled out Sweet Orange for it's foot rot and shallow root system and key lime since it's so sensitve to cold.

I want some cheap practice stocks, since I have enough trouble getting real rootstock seed in my parts. What do you think?

If your keeping them in pots I'll go with lemon. They are very vigorous in there growth also. Have you contacted turtleman as he has a nursery there in arizona and frequents this site.
If not you can try John Watson from the Weslaco Texas citrus center. He might be able to send you seedlings.

For quick rootstock to practice on, I also would choose Lemon. In addition to David's comments, I would add that lemon seedling also tend grow straight. For something to practice grafting, or T-budding, you can use any type of tree, or bush, growing in your yard. - Millet (874-)

I have Flying Dragon seedlings, but there slow growth is a problem. I don't really want to butcher seedlings that take that long to get to proper size.

When you do t budding you only cut off the top only after it's been slowed atleat 14 days to heal. With the time allowed to heal and callous alittle it is usually a 100 percent take unless uncompatabilities.
So its not really so much a butcher job because you only cut the top off when it has healed and the bud took.

Skeet's advice is correct (his advice is always correct), the one caution I would add, when the scion is bent, instead of being cut, is to be careful that while bending it does not crack the root stock. Cracking can happen if not careful. - Millet (872-)

I did t budding about two months ago and the ones I cut the top off are about a foot long. The ones I bent are barely breaking the bud. So in my opinion it's a way to long of a wait for buds to break when bent. It does work but longer for me.

David,on the stocks with bent over tops, you may like to try notching a wound in the stock, above the bud to be forced,( and clear above the final height you will cut the whole top off). This is a halfway method between bending and lopping, and is also useful if you cant bend the top down enough to get the scion bud close to the highest point on the stock after bending. One advantage of bending and forcing is that after the new scion growth gets to be dominant, the bent lower growth can push it harder than if all the stock had been removed. Which method to use depends on the condition of the stocks, type of buds, season etc etc, and personal preference. Recently I took Orange buds from vigorous water shoots, they shot out after two weeks before I even touched the stocks. Other buds from other plants still haven't done much.

Mark-t, lemons are probably the obvious choice as suggested. You may want to plant extra and cull the runts to get vigorous stocks. Otherwise what seeded fruit are available locally ? Mandarins possibly, some vars may not be vigorous enough to be worth the trouble, other mandarin vars may be better. Grapefruit should be the most vigorous, tangelos and sweet orange next. Sour orange vars are also a possibility for use as a stock.