Are you talking about taking
separate quant and verbal sections? Or entire tests?
For entire tests, the two I recommend are our own CATs and GMATPrep.
If you are talking about doing 37-question and 41-question full "sections" by themselves, I don't recommend doing that. If you're going to do a full section, you should do a whole test.
If you are just practicing (even if you're trying to practice timing or stamina), do shorter sections - around 20 questions - and then spend a TON of time reviewing before you do another section. You want to be able to iterate - to get better constantly - so you don't want to waste 40 questions each time. Do shorter sections (and other kinds of study) for a week to two weeks, then take a full practice test when you want to test yourself to see what kind of progress you've made.
For the shorter question sets, make them up yourself from the Official Guide. You want the question sets to be like "mini tests" - so the questions should be chosen randomly (not all in one topic); they should have about an even mix of the different question types (DS and PS, with a few more PS; or SC, CR, and RC, with a few more SC); and they should reflect a range of difficulty levels. In OG, the questions get harder as the numbers get higher, so don't pick 5 or 10 questions in a row; skip around a bit.
Time yourself by adding up the time for the number of questions you've chosen using these guidelines; that's your time limit for your section:
Quant - 2m
SC - 1m15s
CR - 2m
RC - 2m (shorter passage) to 3m (longer passage) to read; 1.5m per question
When you get later in your study, you can also use GMAT Focus for mini quant tests (
www.gmatfocus.com) but I wouldn't use that until you've been through whatever you're using to learn quant at least once and are ready for a good review.