Searching for music loops online can be a herculean task if you are not sure what you are looking for. Drum beats and guitar beats are easy to find, lot of sites offer free versions of these while there are others that offer high quality loops as suites and individual selections for you o download. Few things to look forward to when downloading loops are the quality, bit rate , sample rate, length , tempo etc., to ensure that the loop complies with the quality of your requirement or composition.
Looperman is a site that offers free apple loops, which are in the form of AIFF but there are wav and mp3 file formats are available for use. While this site offers a rather generous offering of loops that range from single instruments to full orchestras, all divided genre and instrument wise which makes it a very well organized site which is easy to use. However the drawback of the site lies in the fact that since these loops are made for a DAW such as logic and are made using the specific DAW which means that there might be time delays and other parameter problems in certain cases which are however very few, since the loops seem to be compliant enough. You can use the loop for your own music as long as you credit the author unlike a lot of other sites which do not let you incorporate loops into your own composition.
Macloops is a paid loop site primarily for mac however alternate studio quality file formats are provided such as acidized wav formats for Sonar etc., there are few hundreds of few loop samples for download however these are what they deem to be- short duration samples. On the other hand there are comprehensive loop packages that are sold genre wise such as orchestral pop, hip hop, cinematic piano loops etc., which can be purchased from the site for a one time direct download and there are virtually all types of instruments and instrumentations downloadable and you own the license to use them in your compositions. The only draws here are that the loop packages are on the higher price side, though in their defense, they are massive, extensive compilations, and of course uniqueness of your sound is compromised since these are available for unmitigated use by anyone who purchases them.
Beat suite, neo sounds are some of the websites that provide stock music tracks and loops that are royalty free meaning that you do not have to pay for each time you use, the only problem here is miscellany and a higher prize end, a problem answered by Xcess music which provides high quality instrumental loops that can be purchased loop wise or suite wise however there is a dearth of variety to an extent.
Royalty free music, template wise and partners in rhyme are some of the free loop provider with a dearth of variety and partners in rhyme is only useful when you are using the whole loop as it is as a back ground score etc., and not as a part of your own composition since the site policy forbids it. This thus is only useful for quick fix sound tracks, small scale commercial back ground scores etc.,
Glooped is a guitar loop site that has some respectable beat loops as well to download and pianoloops is a similar site but the samples are of extremely small size and never more than 5 or 6 bars. Looping such small samples gives a sense of overt repetition but these are extremely useful as connecting and peppered loops that add variety to a lengthier composition.