If you want to try out the major thirds tuning in a quick and inexpensive way, take a set of strings, add a few singles, and fit it on one of your 6-string guitars. For example, you can leave out the D from a standard set and then buy a 0.022 and a 0.030 separately. For a versatile round wound set you can use the following guages as a reference: 0.046 (low E), 0.038 (low Ab), 0.30 (low C), 0.022 (mid E), 0.016 (high Ab), and 0.013 (high C). If you have a 7-string guitar try a 0.010 for the high E.

For a heavier flat wound set suitable for jazz, you can try 0.052 (low E). 0.044 (low Ab), 0.36 (low C), 0.028 (mid E), 0.020 (high Ab), and 0.015 (high C). For the high E on a 7-string you can try a 0.012. I use plain steel for high Ab, high C, and high E, and wound for the bottom four. For the high Ab it is a matter of preference whether you want to go for plain or wound. I have changed my mind a couple of times but I have now settled on a plain Ab with round wound lows and a wound Ab with flat wound lows. If you prefer a very heavy, or very light, set of strings you need to scale the guages up or down appropriately. Generally, it is not too critical to get the gauge exactly right. If you are looking for, say, a 042 string, then both a 040 and a 044 will work fine.

Plain steel strings and round wounds are easy to find on the internet in any gauge you desire but the flat wounds can be a pain. You might have to buy them in batches of five or six rather than as singles, and since they are significantly more expensive than the round wounds you can end up wasting a lot of money on strings you don't like if you want to experiment! I get my strings from Highly Strung in the UK, and dealing with them is fast and hassle-free. If you are based in the U.S. you are spoiled for choice, and you shouldn't have any problems finding an online retailer. I don't want to recommend any of them to you, though, since the two online retailers I contacted in the U.S. both ignored my email requests.

For a classical guitar the string gauges are, of course, very different. I use, from low to high, wound on nylon 028, 031, 036, and 043, and nylon 028, 031, and 039 (my two classical guitars are both 7-string). The only company I have found that will do international orders for individual gauges, rather than sets, for classical guitars is Strings By Mail, and they are great.