As others have said, a longsword really is meant to be used by itself. A shorter, single-hander would be ideal for working with sword and buckler.
For synthetic trainers (far better than wooden wasters, IMHO, and about the same price), I would definitely go with the Purple Heart type-III.
Longsword: $125
http://www.woodenswords.com/ProductDetails.asp?ProductCode=TYPE-III-50Single-hand: $125 ($110 on sale currently)
http://www.woodenswords.com/ProductDetails.asp?ProductCode=TypeIII-ShortIf however you would like to train with steel (which has quickly become the standard in HEMA/WMA circles), there are several great options out there. I think some of the best currently on the market are the Albion Meyer and A&A Fechterspiels, but if you want to spend a little less, the trainers from Darkwood Armory (not to be confused with Darksword Armory, which is totally different) are a pretty good deal. I added the shipping/handling charges on the A&A models since they show it with the price on their pages. I'm not sure about the shipping from the others. Albion's tends to run around $25-$35.
For all steel weapons, I recommend adding a "thrusting tip". One of the easiest methods is to fold a couple of pieces of leather over the tip and tape it down. Or, you can find rubber safety tips online, I think.
Longswords:Albion Meyer: $490
http://albion-swords.com/swords/albion/maestro/sword-practice-meyer.htmA&A Fechterspiel: $490 + $51 S&H (training grade)
http://armor.com/sword203.htmlDarkwood basic-training longsword: $340
http://www.darkwoodarmory.com/index.php?main_page=product_info&cPath=2_6&products_id=87The two following "Federschwerts" are new, so I haven't seen them up-close. But it looks like they're replacing the older Federschwert design that they had, which while light and whippy, were great for light practice without a ton of safety gear. We used to use the old models occasionally with just t-shirts, masks, and gloves, for half-speed, technique-driven work.
CAS-Iberia "club" Federschwert: $295
http://casiberia.com/product/federschwert-club/sr7001CAS-Iberia "Meyer" Federschwert: $299
http://casiberia.com/product/federschwert-meyer/sr7002Single-hands:CAS-Iberia/Hanwei Practical hand-and-a-half: $250
http://casiberia.com/product/practical-hand-and-a-half-sword/sh2106 (I'm including this with the single-hands, since this one is actually a bit small for a longsword, and works nicely for shield or buckler use. Or at least, earlier generations did. I don't think they've changed the design too enormously since then).
CAS-Iberia/Hanwei Practical single-hand: $220
http://casiberia.com/product/practical-single-hand-sword/sh2046Albion I:33 sparring sword: $460
http://albion-swords.com/swords/albion/maestro/sword-practice-I33.htmA&A scholar's sword: $395 + $44 S&H
http://armor.com/sword207.htmlDarkwood Arming sword: $285
http://www.darkwoodarmory.com/index.php?main_page=product_info&cPath=2_6&products_id=503